Unanswered questions concerning Hurricane Isaac

The top winds of Tropical Depression Isaac have fallen to 25 mph, but the storm continues to be a potent rain-maker as it heads north-northwest at 11 mph into Missouri. Isaac has spawned up to 20 suspected tornadoes, brought storm surges as high as 13.6' to the coast (in Lake Borgne, LA), and dumped 20" of rain at one station in New Orleans. The 13.27" of rain that fell at Hattiesburg, MS broke the record for wettest August in the city's history (previous record: 13.03" in 1987.) Major flooding is occurring on seven rivers in Louisiana and Mississippi. Isaac is being blamed for at least four deaths in the U.S., 24 in Haiti, and five in the Dominican Republic.

A few notable rainfall totals from Isaac, through 11 am EDT on Friday:

A major reason for Isaac's heavy rainfall totals has been its very slow motion. This slow speed was due to the fact Isaac has been bumping into a ridge of high pressure that is unusually strong, due to the intense drought over the center of the U.S.; strong drought-amplified high pressure areas are very resistant to allowing any low pressure areas to intrude into their domain. The high pressure area was strong enough this week to allow several all-time records for heat this late in the year to be set:

112° on August 29 at Winner, SD108° on August 29 at Valentine, NE107° on August 29 at Corpus Christi, TX97° on August 29 at Denver, CO (2nd highest so late in the year)

Figure 1. Nighttime view of Hurricane Isaac taken at 1:57 am CDT August 29, 2012, by the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi-NPP satellite. The VIIRS day-night band detects light in a range of wavelengths from green to near-infrared, and uses light intensification to enable the detection of dim signals. In this case, the clouds of Isaac were lit by moonlight. Image credit: NASA.

Isaac's beneficial rains falling in drought-stricken regionsHurricanes get a lot of attention because of the billions in damage they cost, and the lives they disrupt. AIR Worldwide estimated today that insured damage from Isaac would cost up to $2 billion. This does not include damage to infrastructure or uninsured damage, so the final price tag of Isaac's rampage will be more like $3 - $5 billion. However, Isaac is now dumping beneficial rains over Arkansas, Missouri, Illinois, Indiana, Ohio, and Kentucky--regions stricken by the worst drought since the 1950s or 1930s, depending upon the exact location. These regions need 9 - 18 inches of rain to pull them out of drought. Isaac's 3 - 6 inches of rain will not end the drought, but will put a pretty good dent in it. I expect that 3 - 6 inches of rain for a wide swath of prime agricultural land in extreme drought is probably worth at least $5 billion, when you consider that a recent estimate by a Purdue economist put the cost of the great drought of 2012 at more than $77 billion. Only Hurricane Katrina ($146 billion) and the drought of 1988 ($78 billion) have been more expensive disasters, according to NOAA's National Climatic Data Center. Unfortunately, Isaac's arrival is poorly timed, as the storm is arriving during harvest season. The strong winds associated with the storm will flatten many crops, making it more difficult to harvest them, and Isaac's winds may cost farmers several hundred million dollars due to unharvestable crops. Still, the rains from Isaac will be highly beneficial for the success of the upcoming winter wheat season, and for next year's growing season.

Figure 2. Predicted precipitation for the five-day period ending on Tuesday evening shows that Isaac is expected to bring a large region of 3 - 6 inches of rain (red, orange, and brown colors) to Arkansas, Missouri, Illinois, Indiana, Ohio, and Kentucky. Image credit: NOAA/HPC.

Figure 3. The great drought of 2012 has brought so little rain to the Midwest that some areas require over 15" of rain (dark purple colors) to end the drought. Image credit: NOAA/CPC.

Unanswered questions about Hurricane Isaac

1. Did the passage of Hurricane Isaac stir up oil from the Deepwater Horizon oil spill? Isaac was the first hurricane to pass over the site of the 2010 Deepwater Horizon oil spill. We know that large hurricanes are capable of creating currents in deep water at the bottom of the Gulf of Mexico; Hurricane Ivan caused upwelling currents of 0.5 cm/s at a depth of about 500 meters. In an August 28 article in the Huffington Post, Nick Shay, professor of meteorology and physical oceanography at the University of Miami, said: "Winds will push water away from the center of a storm, which causes an upwelling as the ocean tries to adjust. It brings whatever is near the bottom up higher in the water column and currents can then push it towards the coast." Up to 1 million barrels of oil from the spill are estimated to still be present in the deep water sediment, on beaches, and in the marshes of Louisiana, and it is possible some of this oil will wash up on the Gulf Coast in coming months. The storm surge of Isaac also likely flushed out oil lodged in the coastal marshes of Louisiana, but it is unknown how much of a concern this might be.

2. What's the deal with these super-sized Category 1 and 2 hurricanes that have been hitting the U.S.? The past three landfalling hurricanes in the U.S.--Isaac (2012), Irene (2011), and Ike (2008)--have all been exceptionally large, among the top ten on record for horizontal extent of tropical storm-force winds. Each of these storms had an unusually low pressure characteristic of a storm one full Saffir-Simpson category stronger. Is this the new normal for U.S. hurricanes?

3. Did the new $14.5 billion upgrade to the New Orleans levee system cause worse flooding elsewhere? Whenever a new levee or flood control structure is created, you make someone else's flood problem worse, since the water has to go somewhere. Where did the water was stopped by the new $1.1 billion, 1.8 mile-long Lake Borgne flood barrier on the east side of New Orleans go? Did it flow south and contribute to the overtopping of the levees near Braithwaite? Or did it go north and contribute to the 36 hours of storm surge in excess of 5' observed along the Mississippi coast at Waveland? I posed this question to NHC's storm surge expert Jaime Rhome, and he said it was impossible to know without doing detailed storm surge modeling studies.

4. Can only hurricanes beginning with the letter "I" hit the U.S. now? Isaac (2012), Irene (2011), and Ike (2008) are the last three hurricanes to hit the U.S. It turns out that hurricanes that begin with the letter "I" and "C" have more names on the list of retired hurricanes than any other letter (nine each.) I'm thinking Isaac will get its name retired, letting storms beginning with "I" take over sole possession of first place on the retired storms list.

Hurricane Kirk in the Central AtlanticHurricane Kirk intensified into a 105 mph Category 2 hurricane this morning, becoming the 2nd strongest hurricane of the 2012 Atlantic hurricane season. Hurricane Gordon was the only stronger storm; Gordon hit sustained winds of 110 mph just before reaching the Azores Islands on August 18. Kirk has probably peaked in intensity, and is about to move over colder waters and gradually decay. Kirk is not a threat to any land areas.

Figure 4. Morning satellite image of Tropical Storm Leslie.

Tropical Storm Leslie a long-range threat to Bermuda, Canada, and the U.S. East CoastTropical Storm Leslie formed on Thursday in the Central Atlantic. Leslie's formation date of August 30 puts 2012 in 2nd place for earliest formation date of the season's 12th tropical storm. Only 1995 had an earlier formation date of the season's 12th storm. With records dating back to 1851, this year is only the second time 8 total storms have formed in August. The other year was 2004, when the first storm of the season formed on August 1 (Alex), and the 8th storm (Hermine) formed on August 29th. Satellite loops show that Leslie has a modest amount of heavy thunderstorm activity, and respectable low-level spiral bands and upper-level outflow. Conditions appear ripe to allow Leslie to intensify into a Category 2 hurricane by Sunday. Fortunately, Hurricane Kirk is weakening the ridge of high pressure to the north of Leslie, and Leslie is expected to turn to the northwest and miss the Lesser Antilles Islands. However, steering currents for Leslie are expected to collapse early next week, as Leslie gets stuck between two upper level lows. The storm will then slowly meander over the open ocean for many days, potentially threatening Bermuda. Leslie will stay stuck until a strong trough of low pressure approaches the U.S. East Coast around September 8. This trough should be strong enough to pull Leslie to the north and then northeast by September 9. At that time, Leslie may be close enough to the coast that the storm will make landfall in New England, Canada, or the Mid-Atlantic states. Leslie could also miss land entirely; this all depends upon the timing and strength of the September 8 trough of low pressure. Regardless, Leslie is expected to bring an extended period of high waves to the U.S. coast. According to NOAA's Wavewatch III model, large swells from Leslie will reach Bermuda by Monday, and arrive along the U.S. East Coast on Tuesday. These waves will be capable of creating dangerous rip currents and beach erosion.

Portlight disaster relief charity responds to IsaacThe Portlight.org disaster relief charity, founded and staffed by members of the wunderground community, are in Mississippi, helping out with Isaac relief efforts. You can check out their progress or donate to Portlight's disaster relief fund at the portlight.org website.

I'm planning on taking Saturday off, but will have a new post for you on Sunday. Have a great holiday weekend, everyone!

Isaac slamming Gulf Coast with damaging floods, tornadoes

Slow-moving Tropical Storm Isaac continues to hammer coastal Louisiana, Mississippi, Alabama, and Florida's Panhandle with tornadoes, torrential rains, high winds, and a damaging storm surge. Over the past 24 hours, destructive tornadoes have touched down in Biloxi and Pascagoula, Mississippi, and one person was killed by a tree falling on a car in Pearl River County, Mississippi. A major flood event is occurring in Slidell, Louisiana, where Isaac's storm surge filled Bayou Bonfouca and the W-14 Canal, inundating portions of the city with 1 - 5 feet of water. While Isaac is now a weakening minimal-strength tropical storm, it is still a potent rainmaker, and will cause damaging floods all along its path for the next three days. Major river flooding is occurring or is about to occur on a number of rivers in the landfall area. In north central Tangipahoa Parish in southeast Louisiana and southwestern Pike County in southern Mississippi, a mandatory evacuation has been ordered for all low-lying areas and along the Tangipahoa River, due to the potential failure of the Lake Tangipahoa dam. Audubon Park in New Orleans, recorded 11.19" of rain as of 7 pm Wednesday night. An earlier amount of 19" was found to be erroneous, and this is not a 24-hour precipitation record for the city. According to wunderground's weather historian, Christopher C. Burt, New Orleans' greatest 24-hour rainfall on record is 14.01" on July 24 - 25, 1933. The Louisiana official state 24-hour record is 22.00" on Aug. 29, 1962 at Hakberry, although U.S. Army Corps of Engineers `Storm Studies' mentions a 23.80" falling in a 24-hour period at Millers Island during a TS on Aug 7-8, 1940. Storm total was 37.50" over a 60-hour period there during that event.

A few other rainfall totals from Isaac, through 11 am EDT on Thursday:

Isaac's storm surge winds downStorm surge levels along the coast of Mississippi and surrounding areas are gradually receding, and the surge has finally fallen below 5' at Waveland, which experienced a storm surge in excess of 5' for 36 hours. Isaac's storm surge levels were characteristic of a Category 2 hurricane, and lasted for an exceptionally long period of time. Waveland, Mississippi experienced a peak surge of 8' and peak storm tide of 9' (surge plus the natural high tide), which beat the levels that occurred during Category 2 Hurricane Gustav of 2008 (7' of storm tide.) The peak 11.06' storm surge at Shell Beach, which is in Lake Borgne, 30 miles southeast of New Orleans, exceeded the 9.5' surge recorded there during Gustav. According to an article in nola.com, Isaac pushed a storm surge of 13.6' into Lake Borgne, on the east side of New Orleans. This is not far from the 15.5' storm surge Hurricane Katrina brought to the location. It is quite possible that Isaac's storm surge might have breached levees of the east side of New Orleans, flooding areas inhabited by tens of thousands of people, had the Army Corps of Engineers not completed their $14.5 billion upgrade to the New Orleans flood defenses this year. I estimate that storm surge damage from Isaac will exceed $2 billion. Isaac has likely caused $2.5 billion in insured damage not related to flooding, insurance firm Eqecat estimated yesterday. Here were some of the peak storm surge values that were recorded at NOAA tide gauges during Isaac:

Figure 2. A TRMM satellite 3-D view of rainfall on Aug. 28 showed a few very powerful thunderstorms near Isaac's eye were reaching heights of almost 17 km (10.6 miles.) Intense bands of rain around Isaac were occasionally dropping rain at a rate of over 2.75 inches per hour. Image credit: SSAI/NASA, Hal Pierce.

Isaac's storm surge on the Mississippi RiverA storm surge estimated at 12' moved up the Mississippi in Plaquemines Parish near Port Sulphur, LA, near 8:30 pm EDT Tuesday, causing overtopping of the levees and flooding of homes in the mandatory evacuation areas behind the levees. These levees were not part of the $14.5 billion levee upgrade New Orleans got after Hurricane Katrina, and were not rated to Category 3 hurricane strength, like the levees protecting New Orleans are. Since salt water is more dense than fresh water, the surge travelled along the bottom of the river, with the fresh water flow of the river lying on top. The surge continued upriver, and before reaching New Orleans, encountered an underwater barrier in Plaquemines Parish. This barrier was constructed by the Army Corps of Engineers beginning on August 15, in order to keep salt water from moving upstream and contaminating drinking water for Plaquemines Parish and New Orleans. Salt water had made it 90 miles upriver to the outskirts of New Orleans, due to the low flow rate of the river (which had dropped 7' below average in height due to the drought of 2012.) According to a spokesperson for the National Weather Service River Forecast Office, this barrier was probably able to completely block the flow of salt water upriver due to Isaac's storm surge, and no salt water made it as far as New Orleans. However, the massive intrusion of ocean water into the river channel caused the mighty Mississippi's fresh water flow to back up for hundreds of miles. Water levels were elevated by 10' in New Orleans (103 miles upstream from the mouth of the Mississippi), 8' in Baton Rouge (228 miles upstream), and 1.4' at Knox Landing, an amazing 314 miles upstream.

Hurricane Kirk in the Central AtlanticHurricane Kirk intensified into a 75 mph Category 1 hurricane this morning, becoming the busy 2012 Atlantic hurricane season's fifth hurricane. With the season's mid-point of September 10 still almost 2 weeks away, we've already had 12 named storms and 5 hurricanes, which is close to what an entire season experiences in an average year (11 named storms and 6 hurricanes.) Kirk should stay well out to sea and not trouble any land areas.

Figure 3. Morning satellite image of Hurricane Kirk.

Tropical Storm Leslie forms in the Central AtlanticTropical Storm Leslie has formed in the Central Atlantic. Leslie's formation on August 30 puts 2012 in 2nd place for earliest formation date of the season's 12th storm. Only 1995 had an earlier formation date of the season's 12th storm. With records dating back to 1851, this year is only the second time 8 total storms have formed in August. The other year was 2004, when the first storm of the season formed on August 1 (Alex), and the 8th storm (Hermine)formed on August 29th. Leslie is organizing quickly, and appears destined to become a hurricane before the week is out. Fortunately, Hurricane Kirk is weakening the ridge of high pressure to the north of Leslie, and Leslie is expected to turn to the northwest and miss the Lesser Antilles Islands. In the long term, it remains unclear if Leslie will follow Kirk and fully recurve out to sea. The latest 2 runs of the GFS model have predicted that Leslie will recurve out to sea and not threaten any land areas, but the latest 2 runs of the ECMWF model have predicted that the trough of low pressure pulling Kirk to the northeast will not be strong enough to recurve Leslie out to sea. Instead, the ECMWF predicts that a ridge of high pressure will build in early next week, forcing Leslie more to the northwest, making the storm a potential threat to Bermuda, then to the Northeast U.S. and Canada in 8 - 11 days.

Isaac downgraded to tropical storm, but impacts continue

Isaac has been downgraded to a tropical storm after losing some strength as its center is now due west of New Orleans. Isaac continues to inflict tropical storm-force winds along the coast from Louisiana to Alabama. Shell Beach, Louisiana has had sustained tropical storm-force winds since 2pm CDT on Tuesday and as of writing this, they continue to see sustained winds around 39 mph. Radar shows heavy rain is falling from central Louisiana east to Mobile, Alabama. The heaviest rain continues to fall on the east side of the storm. A particularly strong band of thunderstorms is training over Gulfport and Biloxi, Mississippi, extending around the storm to McComb, Mississippi, as well. Satellite imagery shows Isaac still has well-developed outflow at higher levels, and the (albeit messy) structure of the cyclone's core seems to be collapsing this afternoon, which illustrates the weakening trend we've seen in observations and from the Hurricane center.

Figure 1. Isaac as seen from the high-resolution Terra satellite Wednesday at 1:15pm EDT.

Water levels have decreased since this morning's high tide, though impacts will likely continue along the Mississippi River and coastal Mississippi and Alabama. The town of Braithwaite, Lousiana was inundated with water today as storm surge forced the river to spill over its walls. Rescue crews gathered people in boats and rafts, and in some cases, were forced to use axes on rooftops to get the people and pets out of their homes.

Below are some pictures collected from Twitter of Isaac's impacts today.

Isaac pounding Louisiana, Mississippi, Alabama, and Florida

Hurricane Isaac continues to lumber slowly northwestwards at 6 mph, as it pounds Louisiana, Mississippi, Alabama, and Florida's Panhandle with torrential rains, high winds, and a damaging storm surge. The eye was partially over water for most of the 15 hours after Isaac's official landfall at 7:45 pm EDT Tuesday night, but New Orleans radar shows the eye of the storm is now fully ashore near Houma. The radar echoes show some weakening on the west side of the eyewall, where dry air has infiltrated the storm. Wind shear remains light, and upper level outflow over Isaac is as impressive as we've seen so far, with a strong outflow channel to the north, and a respectable one to the south, as well. Infrared and visible satellite loops show a very large, symmetric, and well organized storm, and Isaac is going to be able to stay near Category 1 hurricane strength all day today. This will allow Isaac to drop rainfall amounts of 15 - 20" in some areas of Louisiana before the storm is over. A few rainfall totals from Isaac through 11 am EDT:

A dangerous storm surge event underwayIsaac is bringing a large and dangerous storm surge to the coast from Central Louisiana to the Panhandle of Florida. Late this morning was high tide along much of the coast, and the highest water levels of Isaac are likely being experienced at many locations. At 11:30 am EDT, here were some of the storm surge values being recorded at NOAA tide gauges:

The peak 11.06' storm surge at 1:30 am EDT this morning at Shell Beach, which is in Lake Borgne, 20 miles southeast of New Orleans, exceeded the 9.5' surge recorded there during Category 2 Hurricane Gustav of 2008. In general, the storm surge heights from Isaac have been more characteristic of a strong Category 2 hurricane, rather than the weak Category 1 hurricane one might suppose Isaac is, based on its top sustained winds of 75 - 80 mph. The Saffir-Simpson Scale for ranking hurricanes is only a crude measure of their potential impacts.

A storm surge estimated at 12' moved up the Mississippi in Plaquemines Parish near Port Sulphur, LA, near 8:30 pm EDT Tuesday, causing overtopping of the levees and flooding of homes in the mandatory evacuation areas behind the levees. These levees were not part of the $14.5 billion levee upgrade New Orleans got after Hurricane Katrina, and were not rated to Category 3 hurricane strength, like the levees protecting New Orleans are. The surge continued upriver, elevating the water levels 10' in New Orleans (103 miles upstream from the mouth of the Mississippi), 8' in Baton Rouge (228 miles upstream), and 1.4' at Knox Landing, an amazing 314 miles upstream. The river was 7' low due to the great 2012 U.S. drought, and I suspect the near-record low flow rate of the river allowed the storm surge to propagate so far upstream. The salt water from the storm surge will be slow to leave the river, due to the continued winds of Isaac keeping the surge going, plus the very low flow rates of the river. One benefit of the heavy rains of 10 - 20 inches expected to fall over Louisiana over the next two days will be to increase the flow rate of the Mississippi River, helping flush the salt water out of the river. The low flow rates of the Mississippi had allowed salt water to move upriver to just south of New Orleans over the past few weeks, threatening the drinking water supply of Plaquemines Parish.

Figure 2. Tide gauge data from Waveland, Mississippi. The green line shows the storm surge. The red line is the storm tide, the height of the water above Mean Sea Level (MSL.) The storm tide at Waveland currently (9') is 2' higher than that of Category 2 Hurricane Gustav of 2008. Image credit: NOAA Tides and Currents.

Tropical Storm Kirk in the Central AtlanticTropical Storm Kirk formed Tuesday night in the Central Atlantic. Kirk's formation at 03 UTC on August 29 puts 2012 in 4th place for earliest formation date of the season's 11th storm. Only 2005, 1995, and 1933 had an earlier formation date of the season's 11th storm. Kirk should stay well out to sea and not trouble any land areas.

Figure 3. Morning satellite image of Tropical Storm Kirk.

Invest 98L in the Eastern AtlanticA tropical wave (Invest 98L) is about 750 miles west-southwest of the Cape Verde Islands, and is moving west to west-northwest at about 15 mph. In their 8 am EDT Tropical Weather Outlook, NHC gave 98L a 50% chance of developing by Friday morning. Several of the models develop 98L into a tropical depression by this weekend, but none of the reliable models foresee that 98L will be a threat to the Lesser Antilles. The storm may be a threat to Bermuda next week, but it is too early to say if it may threaten the U.S.

Hurricane Isaac hits Louisiana, driving dangerous storm surges

Hurricane Isaac is ashore over Southeast Louisiana, having officially crossed the coast on the Mississippi Delta 90 miles southeast of New Orleans at 7:45 am EDT on August 28. Isaac intensified right up until landfall, striking with 80 mph winds and a central pressure of 970 mb. The storm's large size and large 50 - 60 mile diameter eye kept the intensification rate slow today, but it came quite close to becoming a significantly more dangerous storm. That's because at landfall, Isaac was in the midst of establishing a small inner eyewall within its large 50-mile diameter eye, a very rare feat I've never seen before. Usually, when an eye first forms, it gradually contracts, eventually becoming so small that it becomes unstable. An outer concentric eyewall then forms around the small inner eyewall, eventually becoming the only eyewall when the inner eyewall collapses. But Isaac is a very unusual storm that has continually surprised us, and this inside-out concentric eyewall formation fits the storm's unusual character. The storm isn't in a hurry to move fully inland, and has slowed down to a crawl this evening. This will give the storm the opportunity to keep its center mostly over water a few more hours, and maintain hurricane strength into the early morning on Wednesday.

Figure 1. Radar reflectivity image from New Orleans as Isaac made landfall at 6 pm CDT August 28, 2012.

A dangerous storm surge event underwayIsaac is bringing large and dangerous storm surge to the coast from Central Louisiana to the Panhandle of Florida. At 10 pm EDT, here were some of the storm surge values being recorded at NOAA tide gauges:

The 9.9' storm surge at Shell Beach, which is in Lake Borgne 20 miles southeast of New Orleans, exceeds the 9.5' surge recorded there during Category 2 Hurricane Gustav of 2008. Research scientists running a Doppler on Wheels radar located on top of the 16' levees in Plaquemines Parish near Port Sulphur, LA, reported at 8:30 pm EDT that a storm surge of 14' moved up the Mississippi River, and was just 2' below the levees. Waves on top of the surge were cresting over the west side of the levee. Needless to say, they were very nervous. Over the past hour, the surge has retreated some, and waves were no longer lapping over the top of the levee. This is probably due to the fact that we're headed towards low tide. A storm surge of 9.5' has moved up the Mississippi River to the Carrrollton gauge in New Orleans. This is not a concern for the levees in New Orleans, since the storm surge has now brought the river up to 2.5' above its normal water level, which was 7' low due to the 2012 U.S. drought. The highest rise of the water above ground level will occur Wednesday morning over much of Southeast Louisiana, Mississippi, Alabama, and the western Florida Panhandle, when the tide comes back in. It is clear now that this storm surge event will be as dangerous as that of Category 2 Hurricane Gustav of 2008. One piece of good news: NWS New Orleans successfully launched their 00Z balloon. However, their discussion noted the atmosphere is "saturated or nearly saturated" all the way up to 470mb, or 20,000 feet. Precipitable water was 2.76 inches, which will be ripe for extremely heavy rainfall.

Figure 2. Tide gauge data from Shell Beach, located on the south shore of Lake Borgne, just east of New Orleans. The green line shows the storm surge. The red line is the storm tide, the height of the water above Mean Sea Level (MSL.)

Portlight disaster relief charity responds to IssacThe Portlight.org disaster relief charity, founded and staffed by members of the wunderground community, have mobilized resources in advance of the arrival of Hurricane Isaac. Their crew, including 2 EMTs, is at the Biloxi Special Needs Shelter, and will be caring for shelter dwellers and doing rescues of people who call for help. Another team will be surveying all the shelters in the area to ensure that they are accessible to all people. You can donate to Portlight's disaster relief fund here.

I'll have more in the morning. Hunker down, New Orleans. It's going to be a long night.

Hurricane Isaac continues to organize as it nears landfall

Isaac gained hurricane status this afternoon after hurricane hunters found winds of 76 mph and a minimum central pressure of 975 mb, and continues to become more organized. In their 5pm EDT update, the National Hurricane Center said Isaac has maximum sustained winds of 80 mph, and is moving northwest at 8 mph--a slow down from yesterday at this time. Satellite loops show that Isaac has gained thunderstorm activity on the southeast side, but is battling dry air to the north. Isaac still appears to be forming an eye wall, though is struggling to do so. Winds have been increasing as Isaac approaches, up to 60 mph has been recorded by buoys close to the shore. Winds off Lake Pontchartrain have varied around 35 mph from the northwest this afternoon. So far New Orleans (KMSY) is seeing wind gusts up to 40 mph, with light rain.

Figure 1. Isaac imagery from the high resolution Terra satellite at 12:30pm EDT, just before becoming a hurricane.

Some rain and thunderstorm activity has reached the northern Gulf Coast this afternoon, with Isaac just over 100 miles southeast of New Orleans. Radar shows thunderstorms from Pensacola to New Orleans. Storm surge has begun to impact the coast, as well, particularly in Mississippi and Louisiana, where reports are that water is "up to cars" in some areas. Many streets are closed in Ocean Springs and Pascagoula, Florida due to surge flooding.

Hurricane Isaac will likely make landfall tonight as a category 1 around midnight CDT, but will still have enough energy and warm water available to maintain hurricane strength for a few hours inland. Isaac will likely continue to strengthen a bit before landfall, but given the large wind field, rapid intensification is unlikely. The storm surge threat could then continue along the Louisiana, Mississippi, Alabama coasts through mid-afternoon on Wednesday, as winds will continue to blow from the east, and then from south, as the storm moves northwest past New Orleans. Should the peak surge coincide with peak tide, the highest storm surge, anywhere from 6 to 12 feet, is expected in Louisiana and Mississippi. Alabama could see a storm surge around 4 to 8 feet.

Figure 2. Rainfall forecast from the HPC through Sunday morning. The lime green bullseye over Gulfport, MS is a value of 15+ inches.

The Hydrometeorological Prediction Center is forecasting around 15 inches of rain to fall around Gulfport, Mississippi, and up to 12 inches of rain to fall near New Orleans, Louisiana by Sunday morning. As Isaac tracks north after landfall, it will bring some much-needed rain to the drought-stricken Mississippi Valley region, particularly west of the Mississippi River. While anywhere from 3-5 inches of rain is expected from Arkansas to Indiana by Sunday, and the Arkansas, Missouri, and Illinois drought will benefit, this is not a drought buster. The driest parts of the country are in the Central Plains states, and this rain will be too little to have a meaningful impact.

Isaac makes its final approach towards Louisiana

The winds and water are rising all along the coast from Louisiana to the Florida Panhandle as Tropical Storm Isaac makes its final approach. Two hurricane hunter aircraft in the storm are measuring a steadily lowering pressure and increasing winds aloft, but hurricane-force winds have not yet been observed at the surface. The 8:30 am center fix found a pressure of 976 mb, which is very low for a tropical storm. Top surface winds measured with the SFMR instrument were 70 mph, but the plane measured 102 mph at an altitude of 5,000 feet. It's more typical to see surface winds of 85 mph with a storm with these characteristics. Infrared and visible satellite loops and hurricane hunter reports from this morning have shown that Isaac has developed a 25-mile diameter eye, though the eyewall has not yet formed a full circle around the eye. Heavy thunderstorm activity is lacking on the north side, where light wind shear of 5 -10 knots is still pumping some dry air into the circulation.

Figure 1. Morning satellite image of Isaac. Note how dry air has wrapped into the west side of the storm, causing a lack of heavy thunderstorm activity.

Isaac's rainsOne of the most remarkable features of Isaac has been the huge spiral band that parked itself along most of the east coast of Florida and remained there for an entire day, despite the fact the center of the storm moved 400 miles away. This rain band was amplified by a weak trough of low pressure along the East Coast, which pulled away from the coast Monday night, taking the band of heavy rain out to sea (except for a few lingering showers near West Palm Beach.) Isaac's heaviest rains fell along a swath from the east coast of Florida near West Palm Beach to the center of the state, just south of Orlando. The 2-day rainfall total of 9.03" at West Palm Beach brought their monthly rainfall total to 22.28", a new August record (old record: 20.12" in 1995.) Vero Beach's 6.48" of rain was a record for any August day. A possible tornado touched down there, damaging 20 mobile homes. In the Keys, rainfall totals as high as 7.94" (at Upper Matecumbe Key) were measured. Heavy rains from Isaac are lingering over Cuba but have ended in Haiti and the Dominican Republic; flash floods in Haiti from Isaac's torrential rains killed at least 24, and two died in the Dominican Republic. The big concern in Haiti is the heavy damage that was done to crops, and the likelihood that the storm's rains will worsen the cholera epidemic that has killed over 7,000 Haitians.

Figure 2. Radar-estimated rainfall from Miami, Florida radar shows that Isaac has dumped a wide swath of 8+ inches of rain (orange colors) across the state. Rainfall amounts in excess of 20" may have fallen just west of West Palm Beach, though the highest amount reported by a rain gauge was 13.10" at Greenacres in Palm Beach County.

Latest model runs for IsaacThe latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs are fairly unified taking Isaac ashore near Southeast Louisiana late Tuesday night or early Wednesday morning, but continue to show some differences in what happens after that. Isaac may scoot nearly west-northwest just inland along the coast into Texas, as predicted by the ECMWF model, or head straight inland to the northwest and into Arkansas, as predicted by the GFS model. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over much of Louisiana. It appears likely that Arkansas will see some heavy rains of up to five inches, which would help put a dent in the exceptional drought conditions there.

Intensity forecast for IsaacLow wind shear of 10 knots or less is likely until landfall, along with very warm ocean temperatures. An analysis of upper level winds from the University of Wisconsin CIMSS shows that Isaac's upper-level outflow is the strongest we've seen, with a solid outflow channel to the south. These conditions favor continued strengthening of Isaac until landfall. However, we've observed in the past many instances of hurricanes suddenly weakening in the final 12 hours before making landfall along the Central Gulf Coast. Katrina, Gustav, Dennis, Ivan, and Rita all did so. A July 2012 paper in the Bulletin of the American Meteorological Society by Rosenfeld et al. titled, AEROSOL EFFECTS ON MICROSTRUCTURE AND INTENSITY OF TROPICAL CYCLONES, theorizes that this may happen because of the impact of small particles that get pulled into the outer circulation of hurricanes, seeding the clouds. These small particles, primarily from air pollution, serve as the seed around which water condenses, increasing the rain in the outer spiral bands. The increase in rain and heat energy at the periphery of the storm comes at the expense of the eyewall and inner core, where the winds tend to weaken. A detailed modeling study by Khain et al. (2010) of Hurricane Katrina in the final day before landfall was able to reproduce the storm's weakening only when this air pollution effect was included. This impact of small particles on hurricanes is not included in any operational hurricane model.

Figure 3. Tide gauge data from Shell Beach, located in Lake Borgne just east of New Orleans. The green line shows the storm surge. The red line is the storm tide, the height of the water above Mean Sea Level (MSL.)

Storm surge observations from IsaacIsaac's storm surge has peaked along the west coast of Florida. As I explain in our Storm Surge Tutorial, we are most interested in the storm tide--the height above Mean Sea Level (MSL) of the tide plus the storm surge. The storm tide is the number given in NHC advisories for how much above ground level the ocean will be at the coast. The storm surge is the extra elevation of the water due to wind blowing on the water, and does not include the action of waves on top of the water, nor the tide. Tide gauges are specially constructed so that transient waves do not impact water level measurements. At Cedar Key on the West Florida coast north of Tampa, a storm surge of 3' and storm tide of 3.8' were observed early this morning. These were the highest water levels measured at any tide gauge along the Florida west coast. Higher storm surges are occurring in the Florida Panhandle. As of 9 am EDT, here were the storm surge/storm tide measurements along the Florida Panhandle:

A storm surge of 3.5 feet was recorded at 10 am EDT at Shell Beach on the east side of New Orleans in Lake Borgne. This site will have one of the highest surge values during Isaac; a storm surge of 9.5' was measured at Shell Beach during Hurricane Gustav in 2008.

Figure 4. Category 2 Hurricane Gustav of 2008 followed a very similar path to Isaac, and brought a storm tide (the combined effect of the storm surge and tidal levels) of up to 14.5' above ground level to the east side of New Orleans. Isaac's surge may be similar, though probably a little less, than Gustav's.

Isaac: similar to Category 2 Hurricane Gustav of 2008 in destructive power?Isaac is a huge and slow-moving storm, with tropical storm-force winds that extend out 205 miles from the center. Isaac has cut its forward speed down from 14 mph yesterday to 10 mph today, and a large swath of the coast will be subject to high winds and a large storm surge for an usually long period of time for a hurricane--up to 24 hours. Long duration winds are much more damaging than short duration winds, and a long duration storm surge event allows damage to occur during multiple high tide cycles. The long duration storm event will also allow very high rainfall totals, resulting in greater fresh-water flooding problems than usual. As a result, I expect Isaac's to cause more damage than the typical Category 1 hurricane. The 9:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 2.3 on a scale of 0 to 6, but the destructive potential of Isaacs's storm surge was 4.1 on a scale of 0 to 6. For comparison, the storm surge destructive potential of Category 2 Hurricane Gustav of 2008 was rated at 4.2 on a scale of 0 to 6, and the wind destructive potential was 1.1--which is lower than Isaac's, even though Isaac was just a tropical storm at 9:30 am EDT. Gustav brought a storm tide (the combined height of the storm surge and high tide) of 14.5' to the east side of New Orleans, and 11' to Waveland, Mississippi. However, the destructive potential of Isaac's surge may be overrated by this analysis. Wave heights this morning from buoy and ships in Isaac have mostly been below 15', which is quite unimpressive. One ship report to the SE of the storm had a 19' wave height (thanks to meteorologist Steve Gregory for pointing this out.) With only another 12 - 18 hours over water, Isaac likely won't have time for its slowing increasing winds to build up a storm surge that will reach as high as 14', like Gustav did. The official NHC forecast of maximum storm surge height of 12' looks like a good one. The highest rainfall total observed in Gustav was 21" at Larto Lake, Louisiana, and I expect we'll exceed that for Isaac, since the storm is moving more slowly. Gustav spawned 41 tornadoes--21 in Mississippi, 11 in Louisiana, 6 in Florida, 2 in Arkansas, and 1 in Alabama. The strongest tornado was an EF2 in Evangeline Parish, Louisiana. Isaac will likely produce 10+ tornadoes. The total damage from Gustav in the U.S. was $4.5 billion (2012 dollars.) I expect Isaac's damage total will be in the $500 million - $4 billion range.

Invest 97L in the Middle AtlanticA tropical wave (Invest 97L) is located in the Middle Atlantic, about 1250 miles west-northwest of the Cape Verde Islands. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 40% chance of developing by Thursday morning. None of the reliable models foresee that 97L will be a threat to any land areas.

Another tropical wave that moved off the coast of Africa on Sunday is located just southwest of the Cape Verde Islands, and is moving west at 15 mph. Several models develop the disturbance into a tropical depression late this week, and NHC is giving the disturbance a 10% chance of becoming a tropical depression by Thursday morning. The disturbance could begin to affect the Northern Lesser Antilles as early as Saturday night, though our two best models, the GFS and ECMWF, predict the center of the disturbance will pass a few hundred miles north of the islands. The disturbance could be a long-range threat to Bermuda.

Isaac approaching hurricane strength

Isaac is walking the line of hurricane status this afternoon after a hurricane hunter mission investigated the storm and found winds of 80+ mph with the SFMR instrument, which looks down at the surface from the plane and estimates what wind speeds are. This instrument has a notoriously rough time in doing so when there's heavy rain, and since the strongest winds were recorded coinciding with the strongest rain, you can imagine that this region of high wind speed could be suspect. The hurricane hunter mission is still in the storm, so I imagine they will issue a special update if needed. Currently the best estimate of wind speed within the storm is 70 mph. Isaac's pressure has been dropping today as well and is now 981 mb. Isaac is moving northwest at 12 mph--no change since this morning. Satellite loops show that Isaac remains large, though asymmetric, with most of the strong thunderstorm activity on the west and southwest side. Isaac's southeast side continues to struggle with dry air and wind shear, which could help to moderate Isaac's intensity as it approaches the coast.

An oil platform in the northern Gulf of Mexico is reporting sustained winds from the north-northeast at 40 mph this afternoon. A buoy west of Tampa, Florida is recording sustained winds around 30 mph, and platforms south of Louisiana are recording winds from 35-40 mph. The widespread heavy rain of yesterday has lightened up in Florida, but a strong line of thunderstorms in one of Isaac's outer bands is training northward along and offshore of the east coast of Florida, affecting everyone from Miami to Jacksonville.

This afternoon the AP reported that Isaac's death toll in Haiti jumped to 19, which puts Isaac's total death count at 21. It appears most of the deaths in Haiti were due to collapsing structures.

Track forecast:Models seem to be coming into better agreement today on where Isaac will make landfall, closing in on Louisiana and New Orleans as most likely landfall point. The ECMWF, HWRF, and UKMET all suggest New Orleans as the landfall location. The GFS is only slightly west of that. The GFDL is the farthest west, predicting landfall near the Louisiana-Texas border. Landfall timing remains Tuesday night. Beyond landfall, Isaac is expected to move north toward the Midwest through the rest of this week, however, models are showing that the system will likely slow down around landfall time, prolonging impacts like surge and inland flooding.

Intensity forecast:The closer Isaac gets to landfall without having formed an eye, the better it is for intensity at landfall. Isaac has strengthened only modestly in the past 24 hours, and is still struggling with a less-than-conducive atmospheric environment. The HWRF remains on the high end of the intensity spectrum, suggesting Isaac will be a weak category 2 upon landfall. Other models suggest it will be a strong category 1, but the difference is splitting hairs. The National Hurricane Center's official forecast is for Isaac to continue strengthening over the next day, reaching category 2 at landfall.

Figure 2. Tide gauge data from St. Petersburg, Florida. The green line shows the storm surge. As Isaac's counterclockwise winds blew offshore this morning, water levels feel two feet at St. Petersburg. The winds switched to onshore this afternoon as the center of Isaac moved more to the northwest, bringing a storm surge of two feet to the city.

Storm surge observations from IsaacThis morning, as Isaac's counter-clockwise winds brought offshore winds to the Tampa/St. Petersburg area, ocean waters fell two feet along the coast. This afternoon, winds have shifted to blow onshore, and a two foot storm surge has been observed at Naples, Fort Meyers, and St. Petersburg on the west coast of Florida. Water levels have also begun to rise along the coast of Louisiana, with a storm surge of 1.5 feet already occurring at Shell Beach on the east side of New Orleans in Lake Borgne.

Little change to Isaac, but storm poses a serious storm surge threat

Tropical Storm Isaac has changed little in strength or organization this morning, as the storm heads northwest at 14 mph towards the Central Gulf Coast. There are two hurricane hunter aircraft in the storm, and Isaac's central pressure held steady at 989 mb at the 8:30 am and 9:15 am center fixes. Top surface winds remain near 65 mph. Infrared and visible satellite loops show that Isaac is a very large storm, but isn't very symmetric. Heavy thunderstorm activity is lacking on the southeast side, where 10 knots of wind shear is driving dry air into the circulation. The center is surrounded by a ring of echoes now, which was not the case on Sunday. However, the echoes are weak. The 8:30 am center report from the Hurricane Hunters reported about half of a ragged eyewall, but the 9:15 am report did not mention any evidence of an eyewall. Isaac will have to form an eyewall in order to intensify significantly.

Figure 1. Morning satellite image of Isaac. Note the lack of heavy thunderstorm activity on the storm's southeast side, where dry air and wind shear are combining to interfere with development.

Isaac's rainsIsaac's heaviest rains have fallen along a swath from the east coast of Florida near West Palm Beach to the center of the state, just south of Orlando. West Palm Beach received 7.57" of rain from Isaac as of 10 am EDT this morning. A trained spotter in Western Boynton Beach reported 10" of rain from midnight to midnight Sunday. Heavy rains from Isaac are lingering over Cuba but have ended in Haiti and the Dominican Republic; flash floods in Haiti from Isaac's torrential rains killed at least nineteen, and two died in the Dominican Republic.

Figure 2. Radar-estimated rainfall from Melbourne, Florida radar shows that Isaac has dumped a wide swath of 3+ inches of rain (orange colors) across the state.

Latest model runs for IsaacThe latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs are fairly unified taking Isaac ashore near Southeast Louisiana late Tuesday night or early Wednesday morning, but continue to show major differences in what happens after that. It is still uncertain if a trough of low pressure passing to the north of Isaac will be able to turn the storm due north, as the ECMWF model is predicting, or bypass the storm, allowing a more west-northwest motion into western Louisiana, like the GFS model is predicting. In either case, Isaac is likely to slow down as it approaches the coast, which will increase the damage potential fro m its wind, storm surge, and rains. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over much of Louisiana. The ECMWF model predicts that these heavy rains will fall more over Mississippi. It appears likely that Arkansas will see some heavy rains from Isaac late in the week, which would help put a dent in the exceptional drought conditions there.

Figure 3. Predicted precipitation for the 8-day period from 2 am Monday August 27 to 2 am Tuesday September 4, from the 2 am EDT August 27 run of the GFS model. This model is predicting a wide swath of 5 - 10 inches of rain (orange colors) will affect portions of Louisiana. Additional very heavy rains are predicted for the Midwest, as moisture from Isaac interacts with a cold front. Image credit: NOAA/NCEP.

Intensity forecast for IsaacIsaac is currently crossing over a relatively cool eddy of water, which will keep intensification slow today. By tonight, the total heat content of the waters increases, which should aid intensification. Low wind shear of 10 knots or less is likely until landfall. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow to the north is not as strong as yesterday, which should also slow intensification today. The models forecast the upper-level outflow should improve by Tuesday. A storm this large will have trouble undergoing rapid intensification, and Isaac's most likely intensity at landfall will be as a Category 1 hurricane, which is what most of the intensity models are forecasting.

Storm surge forecast for IsaacStorm surge is the primary damage threat from Isaac. Isaac is a huge storm, with tropical storm-force winds that extend out 205 miles from the center. For comparison, Hurricane Katrina at landfall had tropical storm-force winds that extended out 230 miles from its center. Isaac's large size will enable it to set a large area of the ocean into motion, which will generate a large storm surge once the storm approaches land on the Gulf Coast. Water levels at Shell Beach, Louisiana, just east of New Orleans, were already elevated by 1' this morning. Conversely, water levels have fallen by 2' this morning at St. Petersburg, Florida, where strong offshore winds due to Isaac's counter-clockwise circulation have carried water away from the coast. The latest 6:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 0.6 on a scale of 0 to 6, but the destructive potential of Isaacs's storm surge was 2.1 on a scale of 0 to 6. I expect this destructive potential will rise above 3 by time Isaac makes landfall, making Isaac's storm surge similar to that generated by Category 2 Hurricane Gustav of 2008, which followed a path very similar to Isaac's predicted path. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. A higher Category 2-scale surge occurred along the south-central coast of Louisiana, and was 12.5' high in Black Bay, forty miles southeast of New Orleans. Recent model runs indicate Isaac may slow down to a forward speed under 5 mph on Tuesday evening and Wednesday morning, close to the coast. If Isaac is just offshore at this time, the coasts of Southeast Louisiana, Mississippi, Alabama, and the Florida Panhandle will be exposed to a large storm surge with battering waves for two high tide cycles. This sort of extending pounding will be capable of delivering more damage than the storm surge of Hurricane Gustav of 2008.

The affect of storm size and angle of approach on storm surgeA 2008 paper by Irish et al., The influence of storm size on hurricane surge, found that large storms like Isaac are capable to delivering a 30% larger storm surge to the coast than a smaller storm with the same maximum wind speeds. The angle with which the storm hit the coast is important, too--a storm moving due north or slightly east of north will deliver a storm surge about 10% greater than a storm moving NNW or NW. Consult our Storm Surge pages for detailed information on what the risk is for the coast. I expect that Isaac's storm surge will be about 30% higher than the typical surge one would expect based on the maximum wind speeds.

Isaac's storm surge will provide the first test of the newly-completed New Orleans levee system upgrade. In the wake of the disastrous storm surge flooding from Hurricane Katrina in 2005, Congress appropriated $14.5 billion to upgrade the New Orleans levee system to withstand a Category 3 hurricane storm surge. Katrina was a Category 3 storm at landfall, but the storm passed far enough to the east of the city that its storm surge was characteristic of a Category 1 - 2 storm at the places where the city's flood walls and levees failed. The new flood defenses were only partially completed in time for the arrival of Hurricane Gustav in 2008, which hit Central Louisiana as a Category 2 storm. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. Since that time, the imposing 2-mile long IHNC Flood Barrier has been completed to block off the funnel-shaped pair of canals on the east side of the city. I expect New Orleans' new flood defenses will be able to hold back Isaac's surge, but areas outside the levees are at risk of heavy storm surge damage.

Figure 5. A portion of New Orleans' new $14.5 billion dollar flood defenses, as taken from an Army Corps of Engineers map.

Invest 97L off the coast of AfricaA tropical wave (Invest 97L) is located in the Middle Atlantic, about 1050 miles west-northwest of the Cape Verde Islands. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Wednesday morning. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands, and high wind shear should begin to tear the disturbance apart on Tuesday.

Another tropical wave that moved off the coast of Africa on Sunday is located just south of the Cape Verde Islands. This disturbance is moving west at 10 - 15 mph, and could arrive in the Lesser Antilles around September 2. Several models develop the disturbance into a tropical depression late this week, and NHC is giving the disturbance a 10% chance of becoming a tropical depression by Wednesday morning.

Angela Fritz will have a new post here by 6 pm EDT. I'm in Atlanta to help out The Weather Channel with their on-air hurricane coverage, and will doing a few 3-minute tropical updates at 30 minutes past the hour between 2:30 - 7:30 pm EDT today.

Isaac becomes better organized, remains a tropical storm

Isaac is gradually becoming better organized this afternoon, with winds of 60 mph and a minimum central pressure of 992 mb. A hurricane warning has been issued for the northern Gulf Coast, including Louisiana and the Florida Panhandle. Strong thunderstorm activity has developed around the inner core of the storm, which is visible on both satellite and radar. This afternoon's Hurricane Hunter mission has found surface winds from 50 mph to 65 mph and steadily dropping pressure--a sign of a strengthening cyclone. Late in the flight, the hunters found a shift in the center to the west, which could either be a trend or just a "wobble," and could readjust. Another mission is on its way to the storm. Upper level outflow remains organized on the north side of the storm, but wind shear remains moderate (10-20 knots) to the south and southeast of Isaac, disrupting the development of organized thunderstorm activity there.

The Sombrero Key, Florida buoy is reporting sustained wind speeds around 50 mph with gusts close to 60. A weather station in Dry Tortugas National Park is reporting sustained wind speeds around 35 mph with gusts up to 45 mph. A tornado watch has been issued for southern Florida, and some storms with weak rotation have been spotted, but no tornado warnings have been issued. Heavy rain is falling in the Keys and southern Florida, and radar-estimated rainfall totals thus far are up to 4 inches. Scattered storms associated with Isaac reach as far north as St. Augustine, Florida this afternoon, and the storm's main rain shield is approaching Orlando.

Track ForecastLike this morning, the models are still disagreeing on exactly where Isaac will make landfall. The ECMWF's track remains the farthest east, and forecasts Isaac to make landfall near the Florida/Alabama border. Next to the west is the UKMET, which is predicting landfall near Mobile, Alabama. The HWRF, GFDL, and GFS are all on the western side of the envelope, with the GFDL and the GFS predicting landfall in western Louisiana, possibly close to the Louisiana/Texas border. In terms of landfall timing, it depends on the track, but we're expecting landfall early Wednesday morning. The National Hurricane Center's 5pm EDT track splits the difference between the GFS model, which is the western solution, and the ECMWF, which is the eastern. They predict Isaac will continue to move northwest over the next day or so before turning slightly to the north before approaching the Louisiana/Mississippi coastline on Tuesday afternoon and evening.

Intensity ForecastThe HWRF model continues to be the most bullish on Isaac's potential intensity, forecasting the storm to reach category 3 status. The GFDL is more reserved and suggest Isaac will only reach strong category 1 wind speeds before making landfall. Given current observations, the National Hurricane Center has backed off the previous forecast that Isaac will intensify to hurricane status around the Florida Keys. They now expect Isaac to remain at tropical storm status as it moves through the central Gulf of Mexico before finally strengthening as it approaches the coast and become a category 2 hurricane just before landfall.

Figure 2. Model forecasts of Isaac's potential track.

Portlight update

"We are coordinating a pro-active response to potential Hurricane Isaac's landfall along the Gulf Coast. This pro-active response is a major step forward for Portlight and would not have been possible without each of you that have been involved over the past four years since our large scale disaster relief effort after Hurricane Ike devastated the Texas Gulf Coast. The response to Hurricane Ike was a profound effort of everyone working together which accomplished a lot of great things including repatriating BillyBadBird, feeding the Bolivar Peninsula, delivering needed supplies to Bridge City, Tx (which was inundated by Ike's surge), providing medical equipment to the disabled, and the heart warming Christmas Party for the residents of Bridge City, Tx. It was truly a group effort and none of it would have been possible if we had not had so many step up, get involved, and provide support and assistance in whatever way each individual could."

Isaac lashing the Keys; an eyewall is building

Tropical Storm Isaac is steadily organizing as it lashes the Florida Keys with heavy rain and tropical storm-force winds. Sustained winds of 44 mph and 41 mph have been observed at Molasses Reef and Sombrero Key, respectively, this morning. Radar out of Key West shows an increase in spiral banding, and the beginnings of an eyewall. An Air Force Hurricane Hunter aircraft completed its first pass through the center of Isaac near 11:30 am EDT, and did not find the pressure had fallen, or that the peak winds had increased. Infrared and visible satellite loops show that Isaac is a large and increasingly well-organized storm. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow is quite good and increasing to the north, but is lacking elsewhere. Moderate wind shear and dry air to the south are interfering with heavy thunderstorm development on Isaac's south side. Heavy rains from Isaac are lingering over Haiti and the Dominican Republic; flash floods in Haiti from Isaac's torrential rains killed at least four people.

Figure 1. Morning reflectivity image from the Radar out of Key West radar shows the northwest section of an eyewall beginning to form to the southeast of the city.

Latest model runs for IsaacThe latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs have diverged significantly, and we can no longer be confident we know where Isaac will make landfall on the Gulf Coast. One camp of models, the UKMET and ECMWF, predict that a trough of low pressure moving across the Southeast U.S. will be strong enough to turn Isaac north to a landfall in the Florida Panhandle. The other set of models, the GFDL, GFS, and HWRF, predict the trough will bypass Isaac, and a ridge of high pressure will build in and force Isaac to a landfall over Louisiana. The official NHC forecast averages out these two extremes, calling for a landfall midway between the two solutions. Odds are, one of the two model solutions will turn out to be the correct one, and the NHC will be forced to make a substantial adjustment in their forecast track to the east or the west. Isaac has the potential to drop torrential rains capable of causing serious flooding and drought relief over the South. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over Southeast Louisiana, where it predicts Isaac will make landfall. The ECMWF model, however, these heavy rains will fall more over the Florida Panhandle, Alabama, and Georgia.

Figure 2. A hurricane forecaster's dilemma: which set of models is correct? The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs have diverged significantly. Our two top models--the GFS and ECMWF--have 72-hour forecasts that are about 350 miles apart. The ECMWF forecast is not shown here, but lies just to the west of the UKMET forecast (white line.)

Intensity forecast for IsaacIsaac survived passage over Hispaniola and Cuba relatively intact. It's large size aided this. Isaac is over very warm waters of 31°C (88°F) with high total heat content in the Florida Straits, but is encountering moderate wind shear of 10 - 15 knots due to upper-level winds out of the southwest. This shear is predicted to relax to the light range tonight as an upper-level anticyclone becomes established over the storm. This should allow for more substantial intensification after Isaac passes the Florida Keys. However, the total heat content of the ocean decreases for Isaac Monday morning as it encounters a relatively cool ocean eddy in the Southeast Gulf of Mexico. If Isaac takes a more westerly track, passing due south of the Central Louisiana coast, the storm will encounter a modest warm eddy, which would aid intensification. The intensify forecasts from the various models are very divergent. The latest 06Z (2 am EDT) run of the GFDL model keeps Isaac as a strong tropical storm until landfall in Louisiana. Isaac will undergo rapid intensification into a Category 3 hurricane as it hits New Orleans, says the latest 06Z (2 am EDT) run of the HWRF model. The ECMWF model has Isaac as a strong Category 2 storm with a central pressure near 950 mb as it hits near the Alabama/Florida border.

Comparing Isaac with Ike of 2008The current situation with Isaac is similar in some ways to that of Hurricane Ike of 2008. Ike spent considerable time over Cuba, weakening from a Category 4 to a Category 1 storm. The storm couldn't put its energy into building a strong inner core, but it was able to build up its outer rainbands that were over very warm waters. This resulted in a major expansion of its wind field, with tropical storm-force winds extending out 275 miles from the center at one point. Ike was able to intensify into a Category 2 storm on its path towards Texas, and had an unusually low pressure for a Cat 2 storm with 100 mph winds--944 mb. That's a central pressure more typical of a Category 3 storm, but Ike could only manage Category 2 winds, since it had such a large chunk of the atmosphere to keep spinning. With Isaac's TS winds already extending out to 205 miles, maybe we'll see another Ike-type situation as it intensifies--the storm will have an unusually low pressure in order to keep a huge wind field spinning, but never make it above Category 2, since it will take so long to spin up such a large wind field.

Storm surge forecast for IsaacIsaac is a very large storm, with tropical storm-force winds that extend out 205 miles from the center. For comparison, Hurricane Katrina at landfall had tropical storm-force winds that extended out 230 miles from its center. Isaac's large size will enable it to set a large area of the ocean into motion, which will generate a large storm surge once the storm approaches land on the Gulf Coast. The latest 3:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 0 on a scale of 1 to 6, but the destructive potential of Isaacs's storm surge was 2.1 on a scale of 1 to 6. A 2008 paper by Irish et al., The influence of storm size on hurricane surge, found that large storms like Isaac are capable to delivering a 30% larger storm surge to the coast than a smaller storm with the same maximum wind speeds. The angle with which the storm hit the coast is important, too--a storm moving due north or slightly east of north will deliver a storm surge about 10% greater than a storm moving NNW or NW. Consult our Storm Surge pages for detailed information on what the risk is for the coast. I expect that Isaac's storm surge will be about 30% higher than the typical surge one would expect based on the maximum wind speeds.

Invest 97L off the coast of AfricaA tropical wave (Invest 97L) is located about 650 miles west of the Cape Verde Islands off the coast of Africa. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 50% chance of developing by Tuesday morning. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands. However, both the GFS and ECMWF models predict that a tropical wave that has not yet emerged from the coast of Africa may develop late this week, and potentially take a more westward track towards the Lesser Antilles, arriving around September 2.

Angela Fritz will have a new post here by 6 pm EDT. For the next few days, I plan to do the morning blog post, and Angela will be doing the late afternoon post. I'm in Atlanta to help out The Weather Channel with their on-air hurricane coverage, and will be on either in the afternoon or evening on Sunday, Monday, and Tuesday.

Isaac remains a tropical storm as it skirts northern coast of southeast Cuba

This afternoon, Isaac remains a tropical storm with little change in intensity as it moves northwest along the northern coast of Cuba, with winds of 60 mph. The storm's center is currently skirting the northern coast of southeast Cuba over the state of Holgiun. There is a hurricane hunter mission currently flying through the storm. The hurricane hunter mission and recent satellite images show that Isaac has maintained its core circulation, providing a launching point for new thunderstorm activity to develop. Isaac's strongest thunderstorms, which were on the southeast side of the storm just south of Hispaniola, have weakened somewhat this afternoon, as stronger thunderstorms begin to develop north of Cuba. A new burst of convection above the storm's center of circulation also began this afternoon. Upper level outflow is strong on the north and northeast side of the storm, though is weak and disorganized on the south and southwest. Wind shear around the storm remains 10-20 knots.

Figure 1. Satellite imagery of Tropical Storm Isaac as it moves northwest along the northern coast of Cuba on Saturday afternoon.

Impacts:Heavy rain continues over Hispaniola this afternoon as the center of Isaac moves away from Haiti. Once person is confirmed dead in Haiti today as a result of Isaac and its torrential rain and flash flooding, though there are reports that up to three are dead in the earthquake and cyclone-ravaged state. Rainfall totals up to 12 inches are possible in Hispaniola when it's all said and done, with local amounts up to 20 inches, especially in the higher elevations of the mountains. Thousands of people have been displaced due to damage to their less-than-substantial tent structures, and schools and other more sturdy buildings have been opened for shelter while the storm continues to produce heavy rainfall over Hispaniola. Rain has begun to fall in the Florida Keys as the storm's far northern thunderstorm activity moves toward the state. Governor Rick Scott has declared a state of emergency for the entire state of Florida in preparation for the impacts of Isaac. The Republican National Convention has at least one attendee backing out so far: Alabama governor Robert Bentley has decided to stay in-state as Isaac approaches. Vice President Joe Biden has also canceled his trip to Tampa which was scheduled for next week alongside the Convention. Amtrak has also shut down part of their Silver Service line from Miami to Orlando.

Track forecast:The GFS and CMC are favoring a track to the Mississippi/Alabama border this afternoon, while the GFDL, HWRF, and ECMWF are all forecasting tracks into the Florida Panhandle. The National Hurricane Center is in agreement with the latter forecast, though it's important not to focus on the center of the forecast track. Tropical Storm Isaac is expected to continue to skirt the northern coast of Cuba throughout the rest of Saturday, at which point it will cross the Florida Keys likely on Sunday, late afternoon. From there, Isaac is expected to move northwest until Tuesday when it will make a slight turn to the north. Landfall in the models has been coming sooner and sooner over the past few days, and is now expected Tuesday evening.

Intensity forecast:In addition to the warm water that Isaac will continue to traverse over the next few days, recent model runs suggest that when Isaac leaves Cuba and enters the Gulf of Mexico, it will be in an environment favorable for intensification. Both the GFS and the ECMWF have been forecasting a large upper level anti-cyclone, or "high," over Isaac as it tracks north through the Gulf. Also, surrounding the upper level high, there are several upper level "lows," that could act to siphon this air away from the storm quickly. These features would provide strong ventilation of the storm, which will likely enhance intensification. The GFDL and the HWRF are in agreement this afternoon that Isaac is likely to reach or come very close category 2 hurricane strength with winds around 95 mph by the time the storm reaches the northern Gulf Coast. The National Hurricane Center is forecasting Isaac to reach category 2 status by 2pm EDT on Tuesday. Given the evidence, I agree with this, and will go a step further to say that a major hurricane (category 3+) is not out of the question, nor is a rapid intensification. Lots of uncertainty remains as Isaac treks across northern Cuba. So many things depend on what form this storm will be in once it leaves that coast for the warm Gulf of Mexico waters.

Portlight gears up for Isaac

Our friends at Portlight Strategies are gearing up for Isaac as we speak. From their blog this afternoon: "John Wilbanks, a long time Portlight member and instrumental in many of the things we do will be heading to Panama City tomorrow afternoon along with a team from Yankee 1, a relief organization made up of retired miltary to provide relief services and SAR if that becomes necessary. John's website, Stormjunkie.com, will be streaming video of operations in the strike zone; you can see what Portlight's teams are doing here. We are also reaching out to the disability community in Haiti and rekindling old relationshps there in the aftermath of Isaac's passage of that island. With over 300,000 still displaced after the earthquake of two years ago, the need there will be great."

Isaac pounding Haiti and the Dominican Republic with torrential rains

Tropical Storm Isaac is pounding Haiti and the Dominican Republic with torrential rains that are causing extremely dangerous flooding and landslides. Isaac's center passed over Haiti's southwest peninsula early this morning, tracking about 50 miles west of the capital of Port-au-Prince. As the center pulled away to the northwest, Isaac's heaviest thunderstorms moved ashore over Hispaniola near sunrise, and are now dumping heavy rains with rainfall rates approaching one inch per hour, according to recent microwave satellite estimates.Barahona on the south coast of the Dominican Republic had received 5.14" of rain as of 8 am EDT this morning, and it is probable that some mountainous areas in Haiti and the Dominican Republic have already received up to 10" of rain from Isaac. These rains will continue though much of the day, and have the potential to cause high loss of life in Hispaniola.

Figure 1. A river north of Port-au-Prince, Haiti in flood due to rains from Isaac. Image from Amélie Baron via Twitter.

Latest observationsIsaac built a partial eyewall last night as the storm approached Haiti, but passage over the rough mountains of Haiti has destroyed the inner core, and the surface center of the storm is now fully exposed to view on satellite images. Radar out of Guantanamo Bay, Cuba shows no sign of an organized center, but does reveal some very intense thunderstorms affecting Eastern Cuba, Western Haiti, and nearby islands. Latest data from the Air Force Hurricane Hunters confirm that Isaac has weakened; during their penetration to obtain their 7:08 am EDT center fix, the aircraft reported top surface winds of 55 mph with their SFMR instrument, top flight-level winds at 5,000 feet of 68 mph, and a pressure rise of 3 mb, to 998 mb. Infrared and visible satellite loops show that Isaac remains a large and well-organized storm, though it lacks an inner core. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow is still good to the north, but is lacking elsewhere. An impressive large multi-day satellite animation of Isaac is available from the Navy Research Lab.

Figure 2. Rainfall rates estimated by the NOAA F-17 polar orbiting satellite at 6:21 am EDT August 25, 2012. Rainfall rates of 1 inch per hour (orange colors) were occurring in a large area to the south of Hispaniola, and these heavy rains have now moved onshore. Image credit: Navy Research Laboratory.

Latest model runs for IsaacThe latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs are similar in spread to the previous set of runs. Our two best models, the GFS and ECMWF, are virtually on top of each other, with a landfall location in the Florida Panhandle between Fort Walton Beach and Panama City. It is likely that the trough of low pressure pulling Isaac to the north will not be strong enough to pull Isaac all the way to the northeast and out to sea, and Isaac has the potential to drop torrential rains capable of causing serious flooding over the Southeast U.S. The latest 8-day precipitation forecast from the GFS model (Figure 3) calls for 10 - 15 inches of rain over portions of Georgia and South Carolina from Isaac. The ECMWF model, however, predicts that a ridge of high pressure will build in and force Isaac to the west after landfall, resulting in a slow motion across the Tennessee Valley into Arkansas by Friday. Arkansas is experiencing its worst drought in over 50 years, so the rains would be welcome there.

Figure 3. Predicted precipitation for the 8-day period from 2 am Saturday August 25 to 2 am Sunday September 2, from the 2 am EDT August 25 run of the GFS model. This model is predicting a wide swath of 5 - 10 inches of rain (orange colors) will affect portions of Cuba, Florida, the Bahamas, and the Southeast U.S. Image credit: NOAA/NCEP.

Intensity forecast for IsaacIsaac survived passage over Hispaniola relatively intact. It's large size aided this, and this will also help it survive passage over Cuba today and Sunday. By the time Isaac pops off the coast of Cuba into the Florida Straits on Sunday, it will likely be a 50 mph tropical storm with a large, intact circulation. Isaac will be over very warm waters of 31°C (88°F) in the Florida Straits, wind shear will be light to moderate, and the upper-level wind pattern will feature an upper-level anticyclone over the storm, aiding its upper-level outflow. As I discussed in my previous post, Crossing Hispaniola and Cuba: a history, there have been five storms since 1900 with an intensity similar to Isaac, which crossed over both Haiti and Cuba, then emerged into the Florida Straits. These five storms strengthened by 5 - 20 mph in their first 24 hours after coming off the coast of Cuba. Given the relatively intact structure of Isaac so far, and the favorable conditions for intensification, I expect Isaac will intensify by 15 - 20 mph in 24 hours once the center moves off of the north coast of Cuba. If Isaac spends a full two days over water after passing the Florida Keys, it is possible that it will have enough time to develop a full eyewall and undergo rapid intensification into a Category 2 or 3 hurricane. The latest 06Z (2 am EDT) run of the GFDL model is calling for Isaac to intensify to Category 2 strength, then weaken to Category 1 at landfall in Mississippi on Tuesday. The 06Z HWRF run is calling for landfall in the Florida Panhandle near Fort Walton Beach as a borderline Category 2 or 3 hurricane. The 5 am EDT NHC wind probability forecast gave Isaac a 19% chance of becoming a Category 2 or stronger hurricane in the Gulf. I expect these odds are too low, and that Isaac has a 40% chance of becoming a Category 2 or stronger hurricane in the Northern Gulf of Mexico. I doubt the storm has much of a chance of hitting Category 4 or 5 status, though. While the surface waters in the Gulf of Mexico are very warm, near 30 - 31°C, the total heat content of these waters is unusually low for this time of year. We got lucky with the Gulf of Mexico Loop Current this summer, as it did not shed a big warm eddy during the height of hurricane season, like happened in 2005 (I discuss this in my Gulf of Mexico Loop Current Tutorial.) Without the type of super-high heat energy we had in 2005 in the Gulf of Mexico, Category 4 and 5 hurricanes in the Gulf of Mexico in 2012 will have difficulty forming.

Invest 97L off the coast of AfricaA tropical wave (Invest 97L) is located about 350 miles west-southwest of the Cape Verde Islands off the coast of Africa. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Monday morning. The 8 am EDT SHIPS model forecast predicts moderate shear for the next 5 days over 97L, so some development is possible if 97L can fend off the dry air to its north. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands. However, both the GFS and ECMWF models predict that a tropical wave that has not yet emerged from the coast of Africa may develop next week, and potentially take a more westward track towards the Lesser Antilles.

The Weather Channel's hurricane expert, Brian Norcross, is now writing a blog on wunderground.com. For those of you unfamiliar with his background, here's an excerpt from his first post, from last night:

"This evening 20 years ago the sun set on the horrendous first day after Hurricane Andrew. I was in downtown Miami at the studios of the NBC station. We knew that there was "total" destruction in South Dade County, but even that didn't describe it. Here's to the people that went through it... and held their families together in a situation that most people can't imagine."

Angela Fritz will have a new post here by 6 pm EDT. For the next few days, I plan to do the morning blog post, and Angela will be doing the late afternoon post.

Crossing Hispaniola and Cuba: a history

Throughout hurricane history, numerous tropical storms and hurricanes have battered themselves against Hispaniola and Cuba. Some have been destroyed; others have survived and gone on to wreak additional havoc. Cuba's most formidable barrier to hurricanes is the one Isaac will be running into--the eastern portion of the island, where mountains up to 6,000 feet high rear up out of the sea. I present here a history of five storms that crossed portions of both Hispaniola and Cuba, similar to Isaac's track. These five storms strengthened by 5 - 20 mph in their first 24 hours after coming off the coast of Cuba, and one went on to become the deadliest disaster in American history--the Great Galveston Hurricane.

Tropical Storm Fay of 2008. This storm was so unpredictable, I nicknamed it "The Joker." Fay got disrupted by passage over Haiti and Eastern Cuba, then slowly intensified to a 50 mph tropical storm as it tracked just south of Cuba. After crossing Central Cuba, Fay intensified from 50 mph to 65 mph in 36 hours over the Florida Straits, before making landfall in southwest Florida. Fay actually strengthened another 5 mph to a 70 mph tropical storm while its center was over land near the western end of Lake Okeechobee.

Hurricane Ernesto of 2006. Ernesto was a hurricane for the briefest of time, just six hours, before it encountered the rugged mountains on the southwest Peninsula of Haiti and Eastern Cuba, which weakened it to a 40 mph tropical storm. After popping off the north coast of Cuba, Ernest had 24 hours over the warm waters of the Florida Straits before making landfall on the southern tip of Florida, but Ernesto was only able to strengthen by 5 mph to 45 mph.

Hurricane Georges of 1998. This nasty Cape Verde hurricane cut a swath of destruction across the Caribbean and in the U.S., killing 602 people, mostly in Haiti and the Dominican Republic. Passage over Hispaniola weakened Georges from a Category 3 storm to a Category 1, and Georges was able to maintain Category 1 status for over a day while traversing the eastern half of Cuba. After the center popped off the coast, Georges had 18 hours over water before it hit Key West, and the hurricane intensified from 85 mph winds to 105 mph winds during that time.

Figure 3. Inside the eye of Hurricane Georges, as seen from a NOAA WP-3D research aircraft on 19 September 1998.

Hurricane Two of 1928. This storm became a Category 1 hurricane with 90 mph winds before reaching Haiti, the got disrupted by close passage to Haiti's southwest peninsula, and Eastern Cuba. After the storm crossed Cuba, it strengthened from 60 mph to 70 mph in the Florida Straits, before close passage by the landmass of South Florida weakened it back to a 60 mph tropical storm again. It eventually made landfall in the panhandle of Florida as a 45 mph tropical storm.

Great Galveston Hurricane of 1900. This deadliest hurricane in American history killed an estimated 8,000 - 12,000 people in Galveston, Texas when it hit as a Category 4 storm with 145 mph winds on September 8, 1900. On its way to Galveston, the storm crossed both Hispaniola and the greater part of the length of Cuba as a tropical storm with 40 - 50 mph winds. When the storm popped out into the Florida Straits, it intensified from a minimum strength 40 mph tropical storm to a 145 mph Category 4 monster in two-and-a-half days. There's a very good chance the hurricane passed over a warm core Gulf eddy on its way to Galveston, allowing explosive deepening to occur. That situation does not exist in the Gulf at present for Isaac.

Figure 4. Aftermath of the Great Galveston Hurricane of 1900 on Galveston Island.

Isaac bearing down on Haiti

Tropical Storm Isaac has remained approximately level in intensity this afternoon. An Air Force Reserve hurricane hunter aircraft measured surface winds of 65 mph at 3:55 pm EDT, with winds at the aircraft's flight level of 5,000 feet of hurricane force, 80 mph. The surface pressure is falling, and is down to 994 mb. Tropical cyclones have a warm core, and the Hurricane Hunters typically find that a storm's lowest pressure is also where the warmest temperature are. That was not the case this morning, but is the case this afternoon. The Hurricane Hunters found a modest 4°C increase in temperature as they penetrated Isaac's core at 2:43 pm EDT (that difference fell to just 1°C by 3:55 pm, though.) A ship located about 100 miles southwest of Santo Domingo in the Dominican Republic measured sustained winds of 58 mph early this afternoon, and wind gusts of 46 mph have been measured on the south coast of the D.R. at Barahona this afternoon. Infrared and visible satellite loops show that Isaac is starting to close off a center, and eyewall formation will likely begin early this evening. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow has improved considerably since this morning.

Figure 1. True-color MODIS satellite image of Isaac taken at 11:10 am EDT August 24, 2012. At the time, Isaac had top sustained winds of 60 mph. Image credit: NASA.

Latest model runs for IsaacThe latest set of 12Z (8 am EDT) model runs is similar in spread to the previous set of runs. Final landfall locations range from Alabama (ECMWF model) to the Florida Panhandle south of Tallahassee (GFS model.) It is possible that the trough of low pressure pulling Isaac to the north may not be strong enough to pull Isaac all the way to the northeast and out to sea, and the ECMWF model indicates that Isaac could stall out after landfall, and spend several days over the Tennessee Valley.

Impact on South Florida, Cuba, Jamaica, and the BahamasResidents of South Florida, Cuba, Jamaica, and the Bahamas should not focus overly much of the exact track of the center of Isaac, because the storm's heaviest winds and rain will be spread out over a large area, and will not be focused near the core of the storm. Tropical storm-force winds currently extend outwards 185 miles to the right of Isaac's center, and will continue to extend outwards about this far as the storm passes by the Bahamas and South Florida. Heavy rains of 4 - 8 inches will be common along this swath. I recommend using the latest wind probability forecast from NHC to see your odds of receiving tropical storm-force winds of 39 mph or higher.

Figure 2. Daily Oceanic Heat Content or Tropical Cyclone Heat Potential (TCHP) for Issac. The forecast points are from the 11 am EDT NHC advisory; the 24 hour forecast point shown here is for 8 am EDT Saturday, and the 72 hour forecast point is for 8 am Monday. For tropical cyclones in favorable environmental conditions for intensification (i.e., vertical wind shear less than 15 kt, mid-level relative humidity >50 %, and warm SSTs [i.e., >28.5C]) and with intensities less than 80kt, values of ocean heat content greater than 50 kJ/cm^2 (yellow and warmer colors) have been shown to promote greater rates of intensity change. Isaac will be in such a region when it is over water between its current location and the Florida Keys. Once Isaac goes beyond the Keys, total ocean heat content will fall to levels not as conducive for rapid intensification. Image credit: NOAA/RAMMB.

Intensity forecast for Isaac in the Gulf of MexicoIsaac will likely be a 50 - 55 mph tropical storm on Saturday and Sunday as it moves over Cuba, but once the storm pops off the coast of Cuba into the Florida Straits, it will likely intensify. Isaac will be over very warm waters of 31 - 32°C (88 - 90°F) in the Florida Straits, wind shear will be light to moderate, and the upper-level wind pattern will feature an upper-level anticyclone over the storm, aiding its upper-level outflow. If Isaac makes landfall near the tip of South Florida, as the latest 12Z (8 am EDT) runs of the GFS, HWRF, and GFDL models are suggesting, the 24 hours it has over water before landfall will probably allow it to intensify by 15 - 20 mph. I think the storm is too large for it to increase its winds more than that in just 24 hours. If Isaac spends a full two days over water after passing the Florida Keys, it is possible that it will have enough time to develop a full eyewall and undergo rapid intensification into a Category 2 or 3 hurricane, though none of the models are currently calling for this to happen. The 5 pm EDT NHC wind probability forecast gave Isaac an 11% chance of becoming a Category 2 or 3 hurricane in the Gulf. While the surface waters in the Gulf of Mexico are very warm, near 30 - 31°C, the total heat content of these waters is unusually low for this time of year. We got lucky with the Gulf of Mexico Loop Current this summer, as it did not shed a big warm eddy during the height of hurricane season, like happened in 2005 (I discuss this in my Gulf of Mexico Loop Current Tutorial.) Without the type of super-high heat energy we had in 2005 in the Gulf of Mexico, I doubt we can get a Category 5 hurricane in the Gulf in 2012.

Invest 97L off the coast of AfricaA tropical wave that moved off the coast of Africa on Thursday has been designated Invest 97L by NHC this morning. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear of 10 - 20 knots. In their 2 pm EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Sunday afternoon. The 2 pm EDT SHIPS model forecast predicts that 97L will track west-northwest over the next few days, and now predicts moderate shear for the next 5 days over 97L. However, the storm will have trouble with dry air, and none of the reliable models currently foresee that 97L will develop over the next five days.

Isaac is strengthening

Tropical Storm Isaac is strengthening. An Air Force Reserve hurricane hunter aircraft measured surface winds of 60 mph on the east side of the center, about 170 miles south of Santo Domingo in the Dominican Republic, at 8:40 am EDT this morning. Winds at the aircraft's flight level of 5,000 feet were hurricane force, 76 mph. The surface pressure remained fairly high, at 1000 mb. Tropical cyclones have a warm core, and the Hurricane Hunters typically find that a storm's lowest pressure is also where the warmest temperature are. However, this morning's flight found that Isaac was still disorganized, with the storm showing almost no evidence of a warm core. Isaac's warmest temperatures were displaced 75 miles to the west of where the lowest pressure was. There were no signs of an eyewall beginning to build. Infrared and visible satellite loops show that Isaac is somewhat asymmetric, with a large band of intense thunderstorms to the east, separated from the core region. This is interfering with both the storm's low-level inflow and upper-level outflow, but the band appears to be dying out. An analysis of upper level winds from the University of Wisconsin CIMSS shows an upper-level outflow channel well-established to the north, and an intermittent outflow channel to the south.

Isaac's rainsRadar imagery from Puerto Rico shows that Isaac is dumping some very heavy rains to the south and east of the center. Ponce, Puerto Rico had a wind gust of 37 mph this morning as a heavy band of rain moved through, and radar-estimated rainfall amounts are in excess of 7 inches for the region just north of Ponce. Power outages to 2,000 homes have been reported in Puerto Rico this morning. NOAA buoy 42085 offshore from Ponce reported a wind gust of 54 mph near 9 am EDT this morning. Rainfall estimates from microwave satellite instruments suggest that Isaac's heaviest rains are to the south of the center, and that the Dominican Republic and Eastern Haiti will escape the worst of Isaac's rains. Haiti's southwest peninsula and Eastern Cuba should suffer the heaviest rains.

Figure 2. Radar-estimated precipitation from the Puerto Rico radar shows the region near Ponce has received up to 7" of rain as of 10 am EDT August 24, 2012.

Latest model runs for IsaacThe latest set of 00Z and 06Z (8 pm and 2 am EDT) model runs have come into better agreement, thanks to the dropsonde mission by the NOAA jet yesterday afternoon and evening. Isaac should move over Haiti's southwest peninsula and then eastern Cuba, then track along the spine of Cuba before popping off into the Florida Straits on Sunday. A trough of low pressure will then pull Isaac to the northwest, and then north, towards the Central Gulf Coast. Landfall locations range from Mississippi (06Z HWRF model run) to the Florida Panhandle south of Tallahassee (06Z GFDL model run.) It is possible that the trough of low pressure pulling Isaac to the north may not be strong enough to pull Isaac all the way to the northeast and out to sea, and the ECMWF model indicates that Isaac could stall out after landfall over the Tennessee Valley for several days.

Intensity forecast for IsaacIsaac has not intensified as much as predicted, and I think that the storm's very large size is partially responsible for that. It's tough to spin up as much atmosphere as Isaac is attempting to do very quickly. Conditions remain favorable for intensification today, with wind shear low, 5 - 10 knots, ocean temperatures warm, 29°C, and dry air mostly mixed out of the storm's core. The large band of intense thunderstorms to the east, separated from the core region, appears to be dying out now, which will help the storm grow more organized. The storm's structure has improved considerably between 9 am - 10 am EDT, with a fairly tight center forming, exposed to view, on the north edge of Isaac's heaviest thunderstorms. A curved band of heavy thunderstorms is now trying to wrap around this center to the northeast, and this band will bring very heavy rains to Haiti and the Dominican Republic this afternoon. I expect that the Hurricane Hunters will observe a partial eyewall in their vortex reports between 2 - 4 pm EDT this afternoon. The storm's large size and disorganized structure suggests that Isaac will be able to intensify only slowly today, and will have top winds of 70 - 75 mph before encountering Southwest Haiti and Eastern Cuba tonight and Saturday. Isaac will likely be a 50 - 60 mph tropical storm on Saturday and Sunday as it moves over Cuba. Once Isaac pops off the coast of Cuba into the Florida Straits, it will be over very warm waters of 31 - 32°C (88 - 90°F), wind shear will be light to moderate. The upper-level wind pattern favorable may be quite favorable for intensification, with low wind shear due to an upper-level anticyclone over the storm--though the models disagree on whether or not this anticyclone will set up directly over Isaac or not. It will probably take at least 24 hours with the storm's center over water for it to become a hurricane. It is possible that Isaac could be approaching Category 3 strength by the time it makes landfall on Tuesday on the Gulf Coast, as suggested by the latest 06Z run of the HWRF model.

Impact on Tampa, FloridaThe Republican National Convention begins on Monday in Tampa, Florida. The latest 11 am EDT wind probability forecast from NHC gives Tampa a 17% chance of receiving tropical storm-force winds and a 1% chance of receiving hurricane-force winds on Monday. Tampa is in the NHC cone of uncertainty, though near the edge of it. At a minimum, Tampa will receive very heavy rains and wind gusts in excess of 40 mph. Isaac is going to be hard-pressed to bring hurricane-force winds to the city, though, since any path that takes it close to Tampa would keep the storm too close to land for significant intensification to occur. I put the odds of a mass evacuation being ordered for Tampa during the convention at 1%. I have detailed information on Tampa's storm surge vulnerability in a post from last week.

Invest 97L off the coast of AfricaA tropical wave that moved off the coast of Africa on Thursday has been designated Invest 97L by NHC this morning. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear of 10 - 20 knots. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Sunday morning. The 8 am EDT SHIPS model forecast predicts that 97L will track west-northwest over the next few days, and encounter a region of high wind shear associated with an upper-level low on Monday and Tuesday. This low may be capable of tearing the storm apart, as happened to Tropical Storm Joyce. None of the models currently foresee that 97L will affect the Lesser Antilles Islands, but 97L may pass near Bermuda 7 - 8 days from now.

20th anniversary of Hurricane AndrewToday, August 24, is the 20th anniversary of Hurricane Andrew, which hit South Florida as a Category 5 hurricane with 175 mph winds--one of only three Category 5 hurricanes ever to hit the U.S. With Isaac churning through the Caribbean this week, I didn't have time to prepare a special post on Andrew, but our Hurricane Andrew archive page has links to satellite and radar images, newspaper headlines, and 49 YouTube videos. Here's an additional link for an Andrew damage video shot by wunderblogger/storm chaser Mike Theiss, when he was 14 years old.

Isaac remains disorganized

Tropical Storm Isaac has strengthened slightly, data from the hurricane hunters show, but the storm remains disorganized and difficult to forecast. If you have to make decisions based upon what Isaac will do, I highly recommend that you wait until at least Friday morning to make a decision, if at all possible, as the forecasts then should be of significantly higher accuracy. Isaac continues to have a large area of light winds about 50 miles across near its center. This makes the storm subject to reformations of the center closer to areas of heavy thunderstorms that form, resulting in semi-random course changes. Until Isaac consolidates, the lack of a well-defined center will make forecasts of the storm's behavior less accurate than usual. An Air Force Reserve hurricane hunter aircraft is in Isaac this afternoon, and has found that surface tropical storm-force winds on the east side of the storm, south of Puerto Rico, have undergone a modest expansion. These winds were mostly in the 40 mph range, with a few areas of 45 mph winds. The surface pressure remained fairly high, at 1004 mb. Infrared and visible satellite loops show that Isaac has fairly symmetric circular cloud pattern, with developing spiral bands that are contracting towards the center, which suggests intensification. However, the storm has a very clumpy appearance, and is a long way from being a hurricane. Given the storm's continued reluctance to organize, Isaac is unlikely to reach hurricane strength before encountering Haiti and Cuba. An analysis of upper level winds from the University of Wisconsin CIMSS shows an upper-level outflow channel well-established to the north, and an intermittent outflow channel to the south. Radar imagery from Puerto Rico shows some weak low-level spiral bands that haven't changed much in intensity or organization this afternoon. NOAA buoy 42060 reported 1-minute mean winds of 35 mph and a wind gust of 40 early this afternoon. At St. Thomas in the Virgin Islands, wind gusts up to 45 mph were observed early this afternoon. Isaac's rains caused major flooding last night in Trinidad and Tobago, the southernmost islands of the Lesser Antilles chain, according to the Trinidad Express. Isaac's rains have not been heavy enough today to cause flooding problems on other islands.

Latest model runs for IsaacThe latest set of 12Z (8 am EDT) model runs have shifted to the west compared to the previous set of runs. The models continue to show a west-northwestward track to a point on the south coast of Hispaniola, then across eastern Cuba and into the Florida Straits between Florida and Cuba. A trough of low pressure is then expected to pull Isaac to the northwest and then north, towards the Florida Panhandle. The big news in this model cycle is that both of our top models--the GFS and ECMWF--predict that 5 - 6 days from now, the trough of low pressure pulling Isaac to the north may not be strong enough to finish the job. These models predict that the trough will lift out and a ridge of high pressure will build in, forcing Isaac more to the west. The GFS predicts this will occur after Isaac makes landfall in the Florida Panhandle, resulting in Isaac moving slowly to the west over land, from Georgia to Alabama. The ECMWF predicts the westward motion will happen while Isaac is in the northern Gulf of Mexico, resulting in an eventual landfall near the Louisiana/Texas border on Thursday. There are some huge issues to resolve to make an accurate long-range track forecast for Isaac. Where will its center consolidate? How will the interaction with the mountains of Hispaniola and Cuba will affect it? Where will Isaac pop off the coast of Cuba? Hopefully, the data being collected by the NOAA jet this afternoon will give us a more unified set of model forecasts early Friday morning. For now, pay attention to the cone of uncertainty. If you're in the cone, you might get hit.

Figure 2. Predicted 5-day rainfall total ending at 8 am EDT Tuesday August 28 from Tropical Storm Isaac. Graphics were generated from the 12Z (8 am EDT) August 23, 2012 run of the HWRF model (top) and GFDL model (bottom). The heaviest rainfall is expected to occur in a 50 - 80 wide wide swath along and to the right of where the center tracks. Amounts in excess of 8 inches (yellow colors) are predicted for portions of the Dominican Republic and Haiti from the HWRF model, but not the GFDL model. Given the current disorganization of Isaac, these rainfall amounts are probably at least 20% too high. Image credit: Morris Bender, NOAA/Geophysical Fluid Dynamics Laboratory.

Little change to Isaac, but intensification coming; Joyce forms

Tropical Storm Isaac is a large and impressive-looking storm on satellite images, but data from the Hurricane Hunters reveal that Isaac remains a minimal-strength tropical storm with 40 mph winds, as it heads westward across the Eastern Caribbean. An Air Force Reserve hurricane hunter aircraft which completed its mission into Isaac at 8 am EDT found top winds at the surface near 40 mph, and highest winds at their 5,000 foot flight level of 47 mph. A NOAA hurricane hunter aircraft flying at 10,000 feet has found top winds of 47 mph at that altitude. The Hurricane Hunters found a broad area of light winds with a central pressure of 1003 mb. The aircraft did not observe an eyewall trying to form, and recent microwave satellite images also show no signs of an eyewall forming. There does not appear to be much in the way of dry air near the core of Isaac, as seen on water vapor satellite loops, which is a big switch from what we've seen previously. Visible satellite loops show that Isaac has a much more symmetric circular cloud pattern, and has developed a Central Dense Overcast (CDO) of high cirrus clouds, the hallmark of an intensifying storm. These clouds have very cold cloud tops, indicating that the updrafts creating them are quite strong. An analysis of upper level winds from the University of Wisconsin CIMSS shows that an upper-level pattern of outflow supportive of significant strengthening has developed this morning, with an upper-level outflow channel now well-established to the north, and a new outflow channel opening to the south. Radar imagery from Puerto Rico shows some weak low-level spiral bands that are slowing intensifying and becoming more organized.

Figure 1. Morning radar image of Isaac taken from the Puerto Rico radar. Isaac's rain bands are weak, but are starting to take on a more spiraling shape.

Intensity forecast for IsaacIsaac has consistently confounded predictions that it would intensify, but all the potential factors inhibiting intensification seem to have diminished to the point where intensification has to occur. Wind shear is low, 5 - 10 knots, and ocean temperatures are warm, 29°C, and these warm waters extend to great depth, giving the storm a high total heat content to work with. These factors, combined with the favorable upper-level outflow pattern and more symmetric cloud pattern, support intensification, and all of the intensity models except the HWRF model predict intensification of Isaac to a strong tropical storm or Category 1 hurricane by Friday afternoon. The latest 8 am EDT run of the SHIPS model predicts that wind shear will be light to moderate, 5 - 15 knots, for the next five days. The 11 am EDT wind probability forecast from NHC predicted a 34% chance that Isaac will become a hurricane by Friday afternoon, and a 6% chance it will be a Category 2 or stronger hurricane then. By Friday afternoon, Isaac will likely be close enough to Southwest Haiti that the inner core will be disrupted, and the storm will likely be a 45 - 55 mph tropical storm on Saturday and Sunday as it moves over Cuba. Once Isaac pops off the coast of Cuba into the Florida Straits, it will be over very warm waters of 31 - 32°C (88 - 90°F), wind shear will be light to moderate, and the upper-level wind pattern favorable for intensification, with low wind shear due to an upper-level anticyclone over the storm. It will probably take at least 24 hours with the storm's center over water for it to become a hurricane.

Impact of Isaac on the IslandsThe south coast of Puerto Rico should see Isaac's heaviest rains and strongest winds beginning near 8 pm EDT tonight, with tropical storm-force winds of 40 - 45 mph potentially affecting the southwest portion of the island. The San Juan airport may be able to stay open during Isaac's passage, but with delays when spiral bands move overhead.

Heavy rains and tropical storm-force winds should arrive on the southern coast of the Dominican Republic late tonight, and the Santo Domingo airport will probably be closed on Friday. Rainfall amounts of 8 - 12 inches will likely affect the Dominican Republic Thursday through Saturday, creating dangerous flash floods and mudslides.

Isaac is potentially a very dangerous storm for Haiti, where 400,000 people still live outside underneath tarps in the wake of the 2010 earthquake. Heavy rains from Isaac will begin on Friday morning in Haiti, and last through Sunday. Rainfall amounts of 8 - 12 inches are possible, which will be capable of causing extreme flooding on the vegetation-denuded slopes of Haiti. It will be a major challenge to keep those Haitians living outside safe, if rainfall amounts of 5 - 10 inches occur.

Impact on Florida, Cuba, Jamaica, and the BahamasFlorida, Cuba, Jamaica, and the Bahamas are all at risk of receiving flooding rains and high winds from Isaac. The latest set of 00Z (8 pm EDT) and 06Z (2 am EDT) model runs for Isaac are fairly unified for the coming three days, showing a west-northwestward track over Southwest Haiti and into Western Cuba. At the 4 - 5 day forecast period for Sunday and Monday, the models have come into better agreement, and have shifted west some. Our best-performing model, the ECMWF, has now shifted Isaac's path more to the east, but still is the westernmost of the models, predicting a landfall for Isaac near the Alabama/Florida border on Wednesday. While we do still have some models predicting a path up the east coast of Florida, model consensus now favors a path up the west coast of Florida through the Gulf of Mexico. The recent reformation of Isaac's center more to the south supports the idea that Isaac will take a track more to the west through the Gulf of Mexico. Since this now means a final landfall for Isaac in the Florida Panhandle is likely, the storm will probably have an extra day over water, increasing the odds that it will become a Category 2 or stronger hurricane before this final landfall. The NOAA jet is scheduled to fly into the storm this afternoon, to do a large-scale dropsonde mission to aid model forecasts. These missions can improve model forecasts by 10 - 20%, so the model runs that will be available early Friday morning should have increased reliability.

Impact on Tampa, FloridaThe Republican National Convention begins on Monday in Tampa, Florida. The latest 11 am EDT wind probability forecast from NHC gives Tampa a 15% chance of receiving tropical storm-force winds and a 1% chance of receiving hurricane-force winds on Monday. The latest model tracks for Isaac suggests that the trough of low pressure pulling the storm to the north will not be strong enough to give Isaac a northeastward component of motion when it crosses Tampa's latitude. Thus, Isaac will have difficulty making a direct hit on Tampa without passing over a considerable amount of land first, making a multi-billion dollar hurricane disaster in Tampa very unlikely. I put the odds of a mass evacuation occurring during the convention at 1%; a limited evacuation of people in the Tampa Bay area living in mobile homes in low-lying areas is probably about 5 - 10 % likely. I have detailed information on Tampa's storm surge vulnerability in a post from last week.

Figure 2. Morning satellite image of Tropical Storm Joyce.

Tropical Storm Joyce forms in the Central AtlanticThe season's tenth named storm of the year, Tropical Storm Joyce, has formed in the Central Atlantic. Joyce's formation on August 23 puts 2012 in a tie for second place with 1995 for earliest formation date of the season's tenth storm. Only 2005 had an earlier appearance of the season's tenth storm, when Tropical Storm Jose formed at 2 pm EDT on August 22. None of the models show that Joyce will be a threat to the Lesser Antilles Islands, but it may be a storm that will affect Bermuda. It is possible that Joyce will complicate the track forecast for Isaac 4 - 5 days from now, when the storms may be close enough together to interact.

Isaac reorganizing as it blows through the Lesser Antilles

Tropical Storm Isaac continues to maintain top winds of just 45 mph as its center prepares to move through the Lesser Antilles islands late this afternoon and early this evening. The entire Lesser Antilles chain of islands is receiving heavy rains from Isaac, with Martinique picking up 1.46" of rain as of 2 pm EDT, and St. Lucia receiving 1.49". However, Isaac is not yet generating much in the way of tropical storm-force winds, and none of the islands had received winds in excess of 30 mph as of 4 pm EDT. During their storm penetration to obtain their 2 pm EDT center fix, an Air Force Reserve aircraft measured top surface winds of just 40 mph, and a central pressure of 1004 mb. Top winds at their 5000 foot flight altitude were 49 mph. Isaac is undergoing significant changes to its structure. The plane found the center had become a broad, elongated oval that extended 40 miles from NW to SE. The old center, fixed at 2 pm near 16.1°N, and closer to the dry air to Isaac's north, is being challenged for dominance by a new center that is attempting to form near a burst of heavy thunderstorms at 15.5°N. The Hurricane Hunters' latest fix at 3:50 pm EDT put the center near 15.9°N, a southwards shift of about 17 miles. The resulting battle between centers is giving Isaac a rather odd spiral rectangular shape, as seen on visible satellite loops. The Hurricane Hunters did not observe an eyewall trying to form, and recent microwave satellite images also show no signs of an eyewall forming. A large area of dry air to the north of the storm, as seen on water vapor satellite loops, continues to interfere with development, and Isaac will not be able to begin strengthening until it resolves the battle between it two competing centers, and casts out the dry air infiltrating it. This is going to take at least a day, since Isaac is a very large storm, and it takes more time to spin up a big chunk of the atmosphere. Radar imagery from Barbados and Martinique show plenty of heavy rain showers, mostly on the south side of Isaac where it is moister. There has been a modest increase in spiral banding since this morning.

Latest model runs for IsaacThe latest set of 12Z (8 am EDT) model runs are very similar to the previous set of runs, which I discussed in detail in this morning's post. The models show a westward track to a point on the south coast of Hispaniola. Isaac's center shift to the south may require some modest adjustments to the south and west for the models. This would result in the storm spending a few hours less time over Hispaniola, and more time over or just south of Cuba. This would slightly decrease the risk to the Dominican Republic, the east coast of Florida, and the Bahamas, but increase the risk to the west coast of Florida. The ECMWF--our best performing model over the past two years--continues to be an outlier among the models. It predicts that Isaac will track just south of Cuba, cross the western tip of Cuba on Monday, then head north towards an eventual landfall in Louisiana. However, this model is keeping Isaac weaker than the other models, and thus predicts the storm will have a weaker response to the trough of low pressure over the Southeast U.S. If the official NHC intensity forecast is right and Isaac becomes a hurricane on Thursday, the more southerly track of the ECMWF is not going to verify, and Isaac will spend considerable time over Cuba on Saturday and Sunday. Where Isaac pops off the coast of Cuba will be critical in determining its future path and intensity, and at this point, we don't know it its more likely that Isaac will go up the east coast of Florida, the west coast, or straight up the peninsula over land. At this point, I'd put the odds at:

40% chance of a track through the Gulf of Mexico, west of Florida25% chance of a landfall in South Florida, and a track mostly over the Florida Peninsula35% chance of a track along the east coast of Florida

Which model should you trust?Wunderground provides a web page with computer model forecasts for many of the best-performing models used to predict hurricane tracks. So which is the best? The best forecasts are made by combining the forecasts from three or more models into a "consensus" forecast. Over the past decade, NHC has greatly improved their forecasts by relying on consensus forecast models made using various combinations of the GFS, GFDL, NOGAPS, UKMET, HWRF, and ECMWF models. If you average together the track forecasts from these models, the NHC official forecast will rarely depart much from it, and the NHC forecast has been hard to beat over the past few years. The single best-performing model over the past two years has been the ECMWF (European Center model). The GFS model has been a close second. You can view 7-day ECMWF and 16-day GFS forecasts on our wundermap with the model layer turned on. Ten-day ECMWF forecasts are available from the ECMWF web site. The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on our computer model forecast page that plots positions from the other major models. As seen in Figure 2, the HWRF and GFDL were well behind the ECMWF and GFS in forecast accuracy in 2011, but were still respectable. The BAMM model did very well at 4 and 5-day forecasts. The UKMET, NOGAPS, and CMC models did quite poorly compared to the ECMWF, GFS, GFDL, and HWRF. For those interested in learning more about the models, NOAA has a great training video (updated for 2011.)

Isaac lashing the Lesser Antilles; TD 10 forms

Tropical Storm Isaac is lashing the entire Lesser Antilles chain of islands with heavy rains this morning, and the winds will be on the increase this afternoon as the storm heads west at 19 mph. Isaac is still a weak and disorganized tropical storm, but that is changing. The Hurricane Hunters are in the storm, and have thus far found no increase in the storm's top surface winds, which remain near 45 mph. In their 7:44 am EDT center fix, an Air Force Reserve aircraft measured a rather unimpressive center pressure of 1007 mb, and top winds at their 1000 foot flight altitude of 56 mph. The plane did not observe an eyewall trying to form, and recent microwave satellite images also show no signs of an eyewall forming. A large area of dry air to the north of the storm, as seen on water vapor satellite loops, continues to interfere with development, and it is unlikely Isaac will become a hurricane today. Satellite loops, however, show that Isaac has developed a Central Dense Overcast (CDO) of high cirrus clouds, the hallmark of a developing storm. These clouds have very cold cloud tops, indicating that the updrafts creating them are quite strong. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow is becoming well-established to the southwest and northwest, but outflow is restricted on the southeast side. A large clump of heavy thunderstorms several hundred miles southeast of the center continues to compete for moisture, and is interfering with the low level inflow and upper level outflow of the storm. The intensification rate of Isaac will increase if the storm is able to integrate this clump of heavy thunderstorms into its circulation, which satellite loops this morning suggest is now beginning to happen. Radar imagery from Barbados and Martinique show plenty of heavy rain showers, mostly on the south side of Isaac where it is moister, but little spiral banding. Hurricane hunter missions are scheduled for Isaac every six hours, and a NOAA hurricane hunter research aircraft will also be in the storm, with missions scheduled every 12 hours. The NOAA jet is first scheduled to fly into the storm on Thursday afternoon, to do a large-scale dropsonde mission to aid model forecasts.

Intensity forecast for IsaacThe latest 8 am EDT run of the SHIPS model shows that wind shear is moderate, 10 - 15 knots, but is expected to relax by this evening to a low 5 - 10 knots. Ocean temperatures have increased to a very warm 29°C, and the storm is now over waters with a high total heat content. The lowering wind shear and warm waters should allow the storm to wall off the dry air that has been interfering with development, and also allow the storm to integrate the thunderstorm clump on its southeast side that has been interfering with low level inflow and upper level outflow. It will take some time for the increase in organization to result in an increase in Isaac's winds, and I still expect top winds of 45 - 60 mph in the Lesser Antilles Islands this evening when the core of the storm moves through. On Thursday, when Isaac will be in the Eastern Caribbean, conditions should be favorable enough to allow steady strengthening to a Category 1 hurricane. The 11 am EDT wind probability forecast from NHC predicted a 47% chance that Isaac will become a hurricane by Friday morning, and a 16% chance it will be a Category 2 or stronger hurricane then. This is a reduction in the odds given in the 5 am advisory.

Impact of Isaac on the IslandsThe entire Lesser Antilles Islands chain will have a three-day period of heavy weather Wednesday through Friday. Sustained tropical storm-force winds extend out about 50 miles to the north of the center and 30 miles to the south, so an 80-mile wide swath of the Lesser Antilles will potentially see tropical storm-force winds of 45 - 60 mph this Wednesday evening. Guadaloupe, Antigua and Barbuda, Dominica, Montserrat, and St. Kitts and Nevis at highest risk of these winds.

Winds in St. Croix in the Virgin Islands will likely rise above tropical storm force on Thursday morning, and the south coast of Puerto Rico should see tropical storm-force winds by Thursday afternoon. The San Juan airport may be able to stay open Thursday afternoon and evening, but I think it is more likely they will be forced to shut down.

On Thursday night, heavy rains and tropical storm-force winds should arrive on the southern coast of the Dominican Republic, and all airports in the D.R. will probably be closed on Friday. Rainfall amounts of 8 - 12 inches will likely affect the Dominican Republic Thursday through Saturday, creating dangerous flash floods and mudslides.

Isaac is potentially a very dangerous storm for Haiti, where 400,000 people still live outside underneath tarps in the wake of the 2010 earthquake. Heavy rains from Isaac will begin on Friday morning in Haiti, and last through Sunday. Rainfall amounts of 8 - 12 inches are possible, which will be capable of causing extreme flooding on the vegetation-denuded slopes of Haiti. It will be a major challenge to keep those Haitians living outside safe, if rainfall amounts of 5 - 10 inches occur.

Impact on Florida, Cuba, Jamaica, and the BahamasFlorida, Cuba, Jamaica, and the Bahamas are all at high risk of receiving hurricane conditions from Isaac. The latest set of 06Z (2 am EDT) model runs for Isaac are fairly unified for the coming three days, showing a westward track to a point on the south coast of Hispaniola. All of the models then predict a more west-northwest track across the island and into eastern Cuba, as Isaac responds to a trough of low pressure over the Southeast U.S. Most of the models then predict a path for Isaac along the spine of Cuba, then into the Florida Straits off the coast of Miami by five days from now. A notable exception is our best-performing model, the ECMWF, which keeps Isaac just south of Cuba, and takes the storm more to the west between Jamaica and Cuba on Saturday, then into the Yucatan Channel between Mexico and Cuba by Monday. However, this model is keeping Isaac weaker than the other models, and thus predicts the storm will have a weaker response to the trough of low pressure over the Southeast U.S. If the official NHC intensity forecast is right and Isaac becomes a hurricane on Thursday, the more southerly track of the ECMWF is not going to verify, and Isaac will spend considerable time over Cuba on Saturday and Sunday. Where Isaac pops off the coast of Cuba will be critical in determining its future path and intensity. Some models predict a more easterly exit point, allowing Isaac to move up the east coast of Florida, and potentially make landfall in the Southeast U.S. The latest 06Z GFS model run predicts a more westerly track, which would potentially allow Isaac to move up the west coast of Florida towards Tampa. Keep in mind that the average error in a 5-day forecast is 260 miles. The two most recent runs of the GFS model, at 00Z and 06Z (8 pm and 2 am EDT), gave positions for Isaac that were about 250 miles apart--the earlier run putting the center near West Palm Beach, and the more recent run giving a location between Key West and Havana, Cuba. While passage over the high mountains of Hispaniola and then Cuba will substantially disrupt Isaac and probably reduce it below hurricane strength, the storm is quite large, and should be able to re-intensify once it emerges over the Florida Straits. Waters will be very warm, near 30°C, wind shear is predicted to be light, and forecasts of the upper-level winds show the possibility of an upper-level outflow pattern very favorable for intensification. If Isaac spends a day over water, that should be enough time for it to intensify into a Category 1 hurricane, and if the storm takes a longer 2-day track over water up either the east or west coast of Florida, a Category 2 or stronger storm is possible.

Isaac is a threat to affect Tampa during the Republican National Convention, August 27 - 30, and the official NHC forecast now has Tampa in the 5-day cone of uncertainly. The latest 11 am EDT wind probability forecast from NHC gives Tampa a 9% chance of receiving tropical storm-force winds for the 24-hour period ending on the morning of first day of the convention (Monday). I blogged about the climatological chances of a hurricane causing an evacuation of Tampa during the convention in a post last week, putting the odds at 0.2%. I put the odds of an evacuation occurring during the convention in the current situation at 3%.

Figure 2. Morning satellite image of Tropical Depression Ten.

Tropical Depression Ten forms in the Eastern AtlanticThe large tropical wave in the Central Atlantic, midway between Africa and the Lesser Antilles Islands, has become Tropical Depression Ten. The depression has an impressive amount of spin and heavy thunderstorm activity, as seen on visible satellite loops, and should be Tropical Storm Joyce by Thursday. None of the models show that TD 10 will be a threat to the Lesser Antilles Islands.

Which model should you trust?Wunderground provides a web page with computer model forecasts for many of the best-performing models used to predict hurricane tracks. So which is the best? Well, the best forecasts are made by combining the forecasts from three or more models into a "consensus" forecast. Over the past decade, NHC has greatly improved their forecasts by relying on consensus forecast models made using various combinations of the GFS, GFDL, NOGAPS, UKMET, HWRF, and ECMWF models. If you average together the track forecasts from these models, the NHC official forecast will rarely depart much from it, and the NHC forecast has been hard to beat over the past few years. The single best-performing model over the past two years has been the ECMWF (European Center model). The GFS model has been a close second. You can view 7-day ECMWF and 16-day GFS forecasts on our wundermap with the model layer turned on. Ten-day ECMWF forecasts are available from the ECMWF web site. The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on our computer model forecast page that plots positions from the other major models. As seen in Figure 3, the HWRF and GFDL were well behind the ECMWF and GFS in forecast accuracy in 2011, but were still respectable. The BAMM model did very well at 4 and 5-day forecasts. The UKMET, NOGAPS, and CMC models did quite poorly compared to the ECMWF, GFS, GFDL, and HWRF. For those interested in learning more about the models, NOAA has a great training video (updated for 2011.)

Hurricane Hunters find tropical storm winds in TD 9

The Air Force Hurricane Hunters are in Tropical Depression Nine, and have discovered a region of 40 - 45 mph winds at the surface, using their SFMR instrument. Flight level winds at their 1000 foot altitude spiked as high as 49 mph. The surface pressure was 1005 mb, a typical one for a weak tropical storm. Based on these measurements, it is highly likely that NHC will name this Tropical Storm Isaac at 5 pm EDT. A large area of dry air lies just to the north of the storm, as seen on water vapor satellite loops, and this dry air is interfering with development, allowing only a few clumps of heavy thunderstorms to fire up near TD 9's center, as seen on visible satellite loops. These loops also show some arc-shaped low clouds expanding away from the heavy thunderstorm area on the south side of the center, showing that TD 9 is ingesting dry air that is creating strong thunderstorm downdrafts, robbing the storm of moisture and energy. An analysis of upper level winds from the University of Wisconsin CIMSS shows that an upper level outflow channel has opened to the southwest, and another channel is attempting to open up to the north. A large clump of heavy thunderstorms several hundred miles to the southeast of TD 9 continues to compete for moisture, and is interfering with the low level inflow and upper level outflow of the storm. The center of TD 9 will pass about 50 miles to the north of buoy 41101 near midnight tonight. The next hurricane hunter mission into TD 9 is scheduled for 2 am Wednesday, and there will be a new mission launched every six hours. The NOAA jet is first scheduled to fly into the storm on Thursday afternoon, to do a large-scale dropsonde mission to aid model forecasts.

Figure 1. Satellite image of TD 9 and 96L taken at 10:45 am EDT August 21, 2012. The two storms are connected by a thin line of low clouds. Image credit: NOAA Environmental Visualization Lab.

Intensity forecast for TD 9The latest 2 pm EDT run of the SHIPS model predicts that wind shear will be low to moderate, 5 - 15 knots, for the next five days. Ocean temperatures will warm from 28°C today to 29°C by Wednesday afternoon, and the total heat content of the ocean will increase sharply during that period. The low wind shear and warm waters will favor strengthening. The main impediment to development through Wednesday will be dry air to the north, though the storm will also have trouble separating from the clump of thunderstorms to its southeast. I expect that TD 9 will continue to struggle with dry air through Wednesday. The official NHC forecast is of a 60 mph tropical storm moving through the Lesser Antilles on Wednesday evening; I put the most likely range of strengths for TD 9 Wednesday evening at 50 - 60 mph. Once TD 9 enters the Eastern Caribbean, wind shear will be low, oceanic heat content high, and the storm should have had enough time to moisten the atmosphere to allow steady strengthening to a hurricane. Intensity forecasts 4 - 5 days out are low in skill, though, and it would not be a surprise at all to see TD 9 struggle like so many other storms have over the past two years, and remain a tropical storm over the next five days. Conversely, rapid intensification into a Category 2 hurricane five days from now, as the official NHC is suggesting may happen, would also not be a surprise.

Figure 2. Daily Oceanic Heat Content or Tropical Cyclone Heat Potential (TCHP) for TD 9. The forecast points are from the 11 am EDT NHC advisory, and the 24 hour forecast point shown here is for 8 pm EDT Wednesday. For tropical cyclones in favorable environmental conditions for intensification (i.e., vertical wind shear less than 15 kt, mid-level relative humidity >50 %, and warm SSTs [i.e., >28.5C]) and with intensities less than 80kt, values of ocean heat content greater than 50 kJ/cm^2 (yellow and warmer colors) have been shown to promote greater rates of intensity change. TD 9 will be crossing into such a region early Wednesday morning, and will enter a region of very high TCHP south of Hispaniola on Friday morning (the 72 hour forecast point.) Image credit: NOAA/RAMMB.

Latest model runs for TD 9The latest set of 12Z (8 am EDT) model runs for TD 9 are fairly unified for the coming three days, showing a west to west-northwest track to a point just south of Hispaniola. Most of the models then predict a more west-northwest track across Southwest Haiti and into eastern Cuba, as TD 9 responds to a small trough of low pressure over the Southeast U.S. A notable exception is our best-performing model, the ECMWF, which keeps TD 9 south of Hispaniola, and takes the storm more to the west over Jamaica by Saturday, and then into the Yucatan Channel between Mexico and Cuba by next Tuesday. We'll have to wait another day to see where the center of TD 9 consolidates before judging which model solution is likely to be correct; reformations of the center closer to bursts of heavy thunderstorm often cause the center point to shift around in the early stages of development, leading to large changes in the forecast track many days later. TD 9 is a threat to affect Tampa during the Republican National Convention, August 27 - 30. I blogged about the climatological chances of a hurricane causing an evacuation of Tampa during the convention in a post last week, putting the odds at 0.2%. The odds in the current situation are higher, probably near 2%. It would take a "perfect storm" sort of conditions to all fall in place to bring TD 9 to the doorstep of Tampa as a hurricane during the convention, but that is one of the possibilities the models have been suggesting could happen.

Disturbance 96L off the coast of AfricaA large tropical wave in the eastern Atlantic about 550 miles southwest of the Cape Verde Islands (Invest 96L) is headed west at 15 mph. This disturbance has an impressive amount of spin, as seen on visible satellite loops, and a respectable amount of heavy thunderstorm activity that is increasing. The storm is under a light 5 - 10 knots of wind shear. Given that TD 9 has moistened up the atmosphere ahead of 96L, this disturbance should have less of a problem with dry air. In their 2 pm EDT Tropical Weather Outlook, NHC gave 96L a 60% chance of developing into a tropical cyclone by Thursday afternoon. This disturbance will track nearly due west the next three days, then is expected to turn more to the west-northwest late this week, bringing it close to the northern Lesser Antilles Islands by Sunday.

Disturbance 95L in the Gulf of Mexico near the Texas/Mexico borderA region of disturbed weather (Invest 95L) is in the Gulf of Mexico, just northeast of Tampico, Mexico. In their 2 pm EDT Tropical Weather Outlook, NHC downgraded the chances of 95L developing to 20%. The Air Force hurricane hunter aircraft scheduled to investigate 95L this afternoon was cancelled. Radar out of Altamira, Mexico does not show any organization to the precipitation echoes and visible satellite loops show that 95L is small and disorganized. The computer models show that 95L should drift westwards, and may move over Mexico on Wednesday.

Tropical Depression Nine approaches the Lesser Antilles

Tropical Depression Nine has formed in the waters a day's journey from the Lesser Antilles Islands, and is headed west towards the islands at 20 mph. This storm could be trouble for much of the Caribbean, and may affect the mainland U.S. next week. TD 9 is under light wind shear of 5 - 10 knots, and is over waters of 28°C. A large area of dry air lies just to the north, as seen on the latest Saharan Air Layer (SAL) analysis and water vapor satellite loops. This dry air is interfering with development, and will keep development slow today. This morning's visible satellite loops show that TD 9's heavy thunderstorm activity is limited on the north side, due to the dry air. TD 9 has yet to develop a good upper level outflow channel, which will also keep development slow today. An analysis of upper level winds from the University of Wisconsin CIMSS shows that the winds at high levels are blowing fairly uniformly from the east over the storm, and these winds would have to take on a more counter-clockwise pattern over TD 9 to produce the most efficient upper-level outflow. The clump of heavy thunderstorms to the southeast of TD 9 is also a limiting factor; this clump of heavy thunderstorms is competing for moisture and interfering with the low level inflow and upper level outflow of the storm. However, if TD 9 manages to wrap in this extra clump of heavy thunderstorms and add their spin to its own spin, it could become a very large and dangerous storm. Heavy thunderstorms are now attempting to fire up around TD 9's circulation center, but this is being hampered by dry air. The center of TD 9 passed about 20 miles to the north of buoy 41040 at 10 am this Tuesday morning. The buoy recorded top winds of just 18 mph this morning, suggesting that this is not yet Tropical Storm Isaac. The first hurricane hunter mission into TD 9 is scheduled for Tuesday afternoon.

Figure 1. Morning satellite image of TD 9.

Intensity forecast for TD 9The latest 8 am EDT run of the SHIPS model predicts that wind shear will be low to moderate, 5 - 15 knots, for the next five days. Ocean temperatures will warm from 28°C this morning to 29°C by Wednesday afternoon, and the total heat content of the ocean will increase sharply during that period. The low wind shear and warm waters will favor strengthening. The main impediment to development through Wednesday will be dry air to the north, though the storm will also have trouble separating from the clump of thunderstorms to its southeast. I expect that TD 9 will continue to struggle with dry air through Wednesday. The official NHC forecast of a 60 mph tropical storm moving through the Lesser Antilles on Wednesday afternoon is on the aggressive side, and I put the most likely range of strengths for TD 9 Wednesday afternoon at 45 - 60 mph. Once TD 9 enters the Eastern Caribbean, wind shear will be low, oceanic heat content high, and the storm should have had enough time to moisten the atmosphere to allow steady strengthening to a hurricane. Intensity forecasts 4 - 5 days out are low in skill, though, and it would not be a surprise at all to see TD 9 struggle like so many other storms have over the past two years, and remain a tropical storm over the next five days. Conversely, rapid intensification into a Category 2 hurricane five days from now, as the official NHC and HWRF model forecasts are suggesting may happen, would also not be a surprise.

Impact of TD 9 on the IslandsThe entire Lesser Antilles Islands chain will have a three-day period of heavy weather Wednesday through Friday, as TD 9 and the associated area of heavy thunderstorms to its southeast passes through. Sustained tropical storm-force winds of 40 - 60 mph will occur in the islands only on Wednesday, with Guadaloupe, Antigua and Barbuda, Martinique, Dominica, Montserrat, and St. Kitts and Nevis at highest risk of these winds.

TD 9 will make its closest approach to the Virgin Islands and Puerto Rico on Thursday, and heavy rains from the storm will affect these islands Thursday through Saturday. Tropical storm-force winds should remain just south of these islands.

On Thursday night, heavy rains and tropical storm-force winds should arrive on the southern coast of the Dominican Republic. If the center of TD 9 remains offshore, as appears likely, heavy rains from TD 9 will still be a danger to the country. Rainfall amounts of 4 - 8 inches will likely affect the Dominican Republic Thursday through Saturday, creating flash floods and mudslides. We can't rule out a direct hit on the Dominican Republic on Thursday evening or Friday morning, since the country is well within the NHC cone of uncertainty.

TD 9 is potentially a very dangerous storm for Haiti, where 400,000 people still live outside underneath tarps in the wake of the 2010 earthquake. Heavy rains from TD 9 will begin on Friday morning in Haiti, and last through Sunday. Rainfall amounts of 5 - 10 inches are possible, which will be capable to causing extreme flooding on the vegetation-denuded slopes of Haiti.

Longer-range outlook for TD 9Heavy rains from TD 9 will begin in Jamaica and eastern Cuba on Friday night, but our vision of where the storm might be headed after Friday gets blurry. The official NHC track forecast at long ranges is heavily weighted towards the GFS and ECMWF models, which have done the best job predicting TD 9's path so far. In general, the models have been predicting a track too far to the north for TD 9, and I expect the storm will remain south of Hispaniola, avoiding the disruptive impact of that island's high mountains. While the current NHC forecast has TD 9 hitting Cuba just north of Jamaica five days from now, keep in mind that the average error in a 5-day forecast is 260 miles. Given the tendency of the models to predict a track too far to the north for this storm, and this season's general steering pattern that has already taken two storms into Mexico's Yucatan Peninsula, we should not be surprised if TD 9 takes a more southerly path than the official NHC forecast, and potentially become a threat to western Cuba or Mexico's Yucatan Peninsula. A trough of low pressure capable of pulling TD 9 to the north enters Western Canada Thursday, and the exact timing and amplitude of this trough as it crosses the U.S. this weekend will determine the ultimate landfall location of TD 9. The ECMWF model has a more northwards position for the trough, and thus keeps TD 9 moving more slowly and father south than the GFS. Both of these models predict an eventual landfall for TD 9 over Florida, and TD 9 is a threat to affect Tampa during the Republican National Convention, August 27 - 30. I blogged about the climatological chances of a hurricane causing an evacuation of Tampa during the convention in a post last week, putting the odds at 0.2%. The odds in the current situation are higher, somewhere in the 1% - 3% range. It would take a "perfect storm" sort of conditions to all fall in place to bring TD 9 to the doorstep of Tampa as a hurricane during the convention, but that is one of the possibilities the models have been suggesting could happen.

Disturbance 95L in the Gulf near the Texas/Mexico borderA region of disturbed weather (Invest 95L) is in the Gulf of Mexico off the coast, just northeast of Tampico, Mexico. The disturbance is due to a trough of low pressure and its associated cold front which moved off the coast over the weekend, but has been fortified via moisture from Tropical Storm Helene, which made landfall Saturday near Tampico. In their 8 am EDT Tropical Weather Outlook, NHC gave 95L a 30% chance of developing into a tropical cyclone by Thursday morning. An Air Force hurricane hunter aircraft is scheduled to investigate 95L this afternoon. Winds at Tampico this morning were light out of the west, suggesting that a surface circulation may be trying to form. Radar out of Altamira, Mexico does not show any organization to the precipitation echoes, though, and visible satellite loops show that 95L is small and disorganized, and some arc-shaped low clouds expanding away from the storm on its north side suggest 95L is ingesting dry air that is creating strong thunderstorm downdrafts, robbing the storm of moisture and energy. The computer models show that 95L should drift westwards over Mexico by Wednesday.

Figure 2. Morning satellite image of 96L.

Disturbance 96L off the coast of AfricaA tropical wave in the eastern Atlantic 450 miles southwest of the Cape Verde Islands (Invest 96L) is headed west at 15 mph. This disturbance has an impressive amount of spin, as seen on visible satellite loops, and a respectable amount of heavy thunderstorm activity. The storm is under a light 5 - 10 knots of wind shear. Given that TD 9 has moistened up the atmosphere ahead of 96L, this disturbance should have less of a problem with dry air. In their 8 am EDT Tropical Weather Outlook, NHC gave 96L a 60% chance of developing into a tropical cyclone by Thursday morning. This disturbance will track nearly due west the next three days, then is expected to turn more to the west-northwest late this week, bringing it close to the northern Lesser Antilles Islands by Sunday.

Typhoon Tembin a threat to TaiwanIn the Western Pacific, we have Typhoon Tembin, which put on a very impressive burst of rapid intensification on Monday, going from a 50 mph tropical storm to a Category 3 typhoon with 125 mph winds in just 24 hours. Tembin is currently a Category 3 storm with 115 mph winds, and is expected to hit Taiwan on Thursday as a Category 2 typhoon. Farther out to sea, Typhoon Bolaven has formed northwest of Guam. Bolaven is expected to become a Category 3 typhoon as it heads toward China, and may pass very close to Taiwan this weekend.

Three Atlantic threat areas may develop; a record fire season for the U.S.

A large tropical wave (Invest 94L) located about 1100 miles east of the Lesser Antilles Islands is headed west at 20 - 25 mph, and is showing increasing organization today. The storm is under light wind shear of 5 - 10 knots, and is over waters of 27°C. A large area of dry air lies just to the north of 94L, as seen on the latest Saharan Air Layer (SAL) analysis and water vapor satellite loops. This dry air is interfering with development, and this morning's visible satellite loop shows that 94L's heavy thunderstorm activity is sparse. However, the satellite loops do show that 94L has now separated from the clumps of heavy thunderstorms to its south, and a pretty well-defined surface circulation has developed. Heavy thunderstorms are now attempting to fire up around this circulation center, but are being hampered by dry air. The center of 94L was about 80 miles to the north of buoy 41041 at 10 am Monday morning, and the buoy recorded SW winds of 10 mph, confirming that 94L probably does have a closed surface circulation. The disturbance will have to build and maintain more heavy thunderstorms than it has now to be considered a tropical depression, though. The first hurricane hunter mission into 94L is scheduled for Tuesday afternoon.

Figure 1. Morning satellite image of Invest 94L.

Forecast for 94LThe latest 8 am EDT run of the SHIPS model predicts that wind shear will be low, 5 - 10 knots, for the next five days. Ocean temperatures will warm from 27°C this morning to 28.5°C by Wednesday morning, and the total heat content of the ocean will increase sharply during that period, as well. The main impediment to development will be dry air to the north, and the SHIPS model predicts the amount of dry air will change little over the next five days. I expect that 94L will continue to struggle with dry air through Wednesday, when it will probably have had enough time to moisten the surrounding atmosphere and protect itself against the dry air. The models have shown increasing unity in taking 94L through the Lesser Antilles on Wednesday, and I expect the storm will be a tropical depression or weak tropical storm with 40 - 50 mph winds at that time. In their 8 am EDT Tropical Weather Outlook, NHC gave 94L a 80% chance of developing into a tropical depression by Wednesday morning. None of the reliable models predict that 94L will reach hurricane strength over the next five days, and it is unlikely that 94L will be able to organize quickly enough to become anything stronger than a 60 mph tropical storm before reaching the Lesser Antilles, given the storm's current struggles with dry air, and the lack of model support for intensification. However, once 94L enters the Eastern Caribbean, wind shear will be low, oceanic heat content high, and the storm should have had enough time to moisten the atmosphere to allow steady strengthening to occur. The main factor that might prevent intensification into a hurricane late this week would be a close pass by the island of Hispaniola. Our top models for long-range 4 - 5 days forecasts all show a path for 94L very close to the island.

Will 94L hit the U.S. mainland?This storm is a long-range threat to the U.S., as historically, 16% of storms in 94L's location have gone on to hit the U.S., with North Carolina the preferred target (10% chance.) A trough of low pressure capable of pulling 94L to the north enters Western Canada Thursday night, and the exact timing and amplitude of this trough will determine the ultimate landfall location of 94L. The long range 7 - 14 day runs of the GFS model over the past three day have all predicted an eventual landfall for 94L in the U.S., though these long-range runs are notoriously unreliable. The predicted landfall locations have ranged from New England to Texas--which isn't much help. The past three runs beginning on Sunday afternoon have all taken 94L over Florida during the August 27 - 29 time frame, which I'm sure is making organizers of the Republican National Convention uncomfortable, since the convention is in Tampa August 27 - 30. However, 94L could miss Florida completely, as the average error in model forecasts going out 7 days is in excess of 500 miles. We can't rule out a North Carolina landfall, but the pattern we've seen so far this year is for landfalls in Mexico's Yucatan Peninsula, so a more southwards path for 94L into the Yucatan is definitely a possibility. Also, we have that huge drought region in the Midwest, which tends to create its own high pressure bubble, which reduces the odds of storms making the turn and hitting the Central or Western Gulf Coast. If 94L makes it to the Western Caribbean, I see the two most likely options as a landfall in Mexico's Yucatan Peninsula (and then westwards into Mexico south of the Texas border), or recurvature into the Florida Gulf Coast.

Figure 2. MODIS satellite image of Hurricane Gordon taken on Sunday August 19, 2012, at 11:55 am EDT. At the time, Gordon was a Category 2 hurricane with 100 mph winds. Image credit: NASA.

Gordon hits the AzoresThe eye of Hurricane Gordon passed over Santa Maria Island in the eastern Azores Islands near 1:30 am EDT this morning. Gordon was a Category 1 hurricane with 75 - 80 mph winds at landfall. Winds at the Santa Maria airport reached a sustained 49 mph at 3 am EDT, but the airport did not report winds during passage of the eyewall at 1:30 am. Reuters reported that Gordon caused only minor flooding and power outages. The hurricane is being sheared apart by strong upper-level winds, and the extratropical remnants of Gordon will not bring any strong winds or significant rain to Europe.

Disturbance 95L in the Gulf near the Texas/Mexico borderA region of disturbed weather (Invest 95L) has developed in the Gulf of Mexico off the coast, just northeast of Tampico, Mexico. The disturbance is due to a trough of low pressure and its associated cold front which moved off the coast over the weekend, but has been fortified via moisture from Tropical Storm Helene, which made landfall Saturday near Tampico. If 95L were to develop into a tropical storm, it would receive a new name. In their 8 am EDT Tropical Weather Outlook, NHC gave 95L a 30% chance of developing into a tropical cyclone by Wednesday morning. An Air Force hurricane hunter aircraft is scheduled to investigate 95L this afternoon. Winds at Tampico this morning were light out of the northeast, which implies that no surface circulation is forming at this time. Radar out of Altamira, Mexico does show some banding to the precipitation echoes, though, which may be indicative of something trying to spin up. The computer models show that 95L should move little over the next few days.

Figure 3. Radar out of Altamira, Mexico at 9:45 am EDT August 20, 2012, shows some banding to the precipitation echoes in association with 95L.

Disturbance 96L off the coast of AfricaThe tropical Atlantic is very busy this third week of August, and this is the week of the year that we typically see a major ramp-up of tropical storm activity in the Atlantic. A new tropical wave that emerged from the coast of Africa Sunday (Invest 96L) is headed west at 15 - 20 mph. This disturbance has a modest amount of spin and heavy thunderstorm activity, and is under a moderate 10 - 20 knots of wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 96L a 20% chance of developing into a tropical cyclone by Wednesday morning. This disturbance does not have much model support for development.

Figure 4. The new Suomi National Polar-orbiting Partnership satellite (S-NPP) carries an instrument so sensitive to low light levels that it can detect wildfires in the middle of the night. On August 17, 2012, the Visible Infrared Imaging Radiometer Suite (VIIRS) on Suomi-NPP acquired this image of the wildfires blazing in Idaho. The images were created with data from the instrument’s "day-night band," which sensed the fire in the visible portion of the spectrum. The Halstead Fire, centered about 18 miles northwest of Stanley, was sparked by lightning on July 27, and is burning in an area with large numbers of trees killed by the mountain pine beetle. As of Sunday afternoon, the fire had burned 92,000 acres was only 5% contained, according to InciWeb. The fire prompted the evacuation of the town of Featherville on Saturday night. Red flag warnings for adverse fire weather were posted in the region yesterday, and temperatures reached the low 90s with 16% humidity and winds of 10 mph. Image credit: NASA.

A record fire season in the U.S.Massive fires continue to burn in Nevada, Idaho and California, and fires that are currently active in the Western U.S. have consumed over 1.3 million acres of land--an area approximately the size of Delaware. Thanks to widespread drought and unusually high temperatures over the past month, 3 million acres have gone up in flames since mid-July, and the fire season of 2012 now ranks in first place for the most acreage burned at this point in the year. According to the Interagency Fire Center, 6.8 million acres have burned as of August 19 this year, beating the previous record set just last year (6.5 million acres for the year-to-date period.) The Interagency Fire Center shows year-to-date records just for the past ten years. The 2012 fire season is well ahead of the pace of 2006, which was the worst fire year in the U.S. for total acreage burned in a year (records began in 1960). In 2006, 9.9 million acres burned, and 6.4 million acres had burned by August 19. With drought conditions far more widespread this year compared to 2006, and the latest forecasts calling for little drought relief over the coming two months, 2012 is likely to surpass 2006 as the worst fire year in U.S. history before the end of the year.

Figure 5. Comparison of drought conditions between the previous record fire year in the contiguous U.S. (2006) with 2012. Drought is much more widespread in 2012 compared to 2006, and 2012 will likely finish ahead of 2006 for the most acreage burned since record keeping began in 1960. Image credit: U.S. Drought Monitor.

Global warming expected to increase fire activity in the Western U.S.As I blogged about in June, the severe fire seasons of 2012 and 2011 fit the pattern of what we expect to see more of with global warming. Hotter heat waves dry out vegetation more readily, resulting in increased probability of more acreage burned. A study published in the Journal Ecosphere in June 2012 used fire models driven by the output from sixteen climate models used in the 2007 IPCC report and found that while 8% of the planet should see decreases in fire activity over the next 30 years, 38% should see increases. By the end of the century, 20% of the globe should see decreased fire activity, and 62% increased fire activity. In the U.S., the regions most at risk of increased fires are the tundra regions of northern Alaska, and the West, with Arizona and Colorado at particularly high risk.

94L still disorganized; Hurricane Gordon bearing down on the Azores

A tropical wave (Invest 94L) located midway between the Lesser Antilles Islands and the coast of Africa is headed west at 20 - 25 mph. This storm is a threat to develop into a tropical storm that will affect the Lesser Antilles Islands as early as Wednesday. The storm is under light wind shear of 5 - 10 knots, and is over waters of 27°C. A large area of dry air lies just to the north of 94L, as seen on the latest Saharan Air Layer (SAL) analysis and water vapor satellite loops. This dry air is interfering with development, and this morning's visible satellite loop shows that 94L's heavy thunderstorm activity is a bit sparse. The storm does have an impressive amount of spin at middle levels of the atmosphere, though. A pass from the Indian OceanSAT-2 satellite Saturday night at 10:06 pm EDT noted a broad, elongated center of nearly calm winds several hundred miles in diameter at the surface, and nothing resembling a well-organized closed surface circulation. 94L will pass near buoy 41041 on Monday morning. The first hurricane hunter mission into 94L is scheduled for Tuesday afternoon.

Figure 1. Morning satellite image of Invest 94L.

Forecast for 94LThe latest 8 am EDT run of the SHIPS model predicts that wind shear will be low, 5 - 10 knots, and ocean temperatures will be near 27°C through Monday, then warm to 28°C by Tuesday night. As is typical with storms making the crossing from Africa to the Antilles, dry air to the north will likely interfere with development, and the SHIPS model predicts increased dry air as 94L approaches the Lesser Antilles on Wednesday. However, with shear expected to be low, dry air may be less of an issue for 94L than it was for Ernesto or TD 7. Our two best performing models--the GFS and ECMWF--have both been taking 94L through the Lesser Antilles with every run for the past 36 hours. Both models continue to agree on the timing, with 94L arriving Wednesday night or Thursday morning. The BAMM model, which performed as well as the ECMWF and GFS at 5-day forecasts in 2011, is showing a track just north of the Lesser Antilles. Given this agreement among our top three models for long-range forecasts, I give a 60% chance that 94L will pass through the Lesser Antilles. In their 8 am EDT Tropical Weather Outlook, NHC gave 94L a 60% chance of developing into a tropical depression by Tuesday morning. The GFS model has backed off on its forecast that 94L will develop into a hurricane before reaching the islands, and is now predicting 94L will be a tropical storm with 40 - 50 mph winds at that time. The ECMWF model does not develop 94L. It is unlikely that 94L will be able to organize quickly enough to become anything stronger than a Category 1 hurricane before reaching the islands, given the storm's current struggles with dry air, and the lack of model support for intensification. With 94L staying relatively weak and disorganized, the chances of it turning to the northwest and missing the Lesser Antilles, as the NOGAPS model has been predicting, are diminishing. The GFS model predicts that 94L will go on to hit the Dominican Republic as a strong tropical storm on Friday, though the storm could also miss the island, passing just to the north or the south. At longer ranges, the storm is capable of going anywhere from Canada to Mexico's Yucatan Peninsula; it's too early to tell.

Figure 2. The 00Z (8 pm EDT) run of the GFS model from August 18, 2012, was done 20 different times at low resolution using slightly different initial conditions to generate an ensemble of forecasts (pink lines.) The high-resolution operational GFS forecast is shown in white. The GFS ensemble forecast is showing decreasing risk to the U.S. East Coast at long ranges, and an increasing risk to the Gulf Coast.

Gordon bears down on the AzoresHurricane warnings are flying for the central and eastern Azores Islands as Hurricane Gordon barrels eastwards at 21 mph. Gordon's peak 110 mph winds it had last night made it the strongest hurricane of the 2012 Atlantic hurricane season so far. Latest visible satellite loops show that cold water and high wind shear are taking a toll on Gordon, with the southern portion of the storm deteriorating and the eye beginning to open up. However, Gordon will still be a Category 1 hurricane or strong tropical storm when it passes through the Azores Monday morning. Winds at Ponta Delgada were 11 mph out of the east at 10 am EDT this morning, but will rise through the day as Gordon approaches.

Gordon is not a threat to any other land areas, and the extratropical remnants of Gordon will not bring any strong winds or significant rain to Europe. The last time the Azores were affected by a tropical storm was in 2009, when Tropical Storm Grace brought 65 mph winds on October 4. No significant damage was reported. Ironically, the last hurricane to affect the Azores was the 2006 version of Hurricane Gordon, which caused minor damage in the Azores, consisting of mostly fallen trees and power outages. However, after Gordon became an extratropical low, four injuries due to falling debris from high wind were reported in Spain, and Gordon brought high winds and rain that affected practice rounds at the Ryder Cup golf tournament in Ireland.

Figure 3. MODIS satellite image of Hurricane Gordon taken on Saturday August 18, 2012, at 11:50 am EDT. At the time, Gordon was a Category 1 hurricane with 80 mph winds. Image credit: Navy Research Lab, Monterey.

July 2012: Earth's 4th warmest; update on 94L--a threat to the Antilles

July 2012 was the globe's 4th warmest July on record, according to the National Oceanic and Atmospheric Administration's National Climatic Data Center (NCDC).NASA rated it the 12th warmest. July 2012 global land temperatures were the 3rd warmest on record, breaking a streak of three months (April, May, and June) when global land temperatures were the warmest on record. July 2012 global ocean temperatures were the 7th warmest on record, and it was the 329th consecutive month with global temperatures warmer than the 20th century average. The last time global temperatures were below average was February 1985. Global satellite-measured temperatures in July for the lowest 8 km of the atmosphere were 7th or 5th warmest in the 34-year record, according to Remote Sensing Systems and the University of Alabama Huntsville (UAH). Wunderground's weather historian, Christopher C. Burt, has a comprehensive post on the notable weather events of July in his July 2012 Global Weather Extremes Summary.

Figure 1. Departure of temperature from average for July 2012. Most areas of the world experienced much higher-than-average monthly temperatures, including most of the United States and Canada. Meanwhile, Australia, northern and western Europe, eastern Russia, Alaska, and southern South America were notably cooler than average. Image credit: National Climatic Data Center (NCDC) .

El Niño watch continuesSea surface temperatures increased to 0.8°C above average as of August 13 in the equatorial Pacific off the coast of South America. Ocean temperatures have been near or above the 0.5°C above average threshold needed for a weak El Niño event since the beginning of July. However, winds, pressures, and cloud cover over the region have not responded in the fashion typically associated with an El Niño, and NOAA's Climate Prediction Center (CPC) said that "The lack of a clear atmospheric response to the positive oceanic anomalies indicates ongoing ENSO-neutral conditions," in their August 9 El Niño discussion. They have issued an El Niño watch, and give a 71% chance that an El Niño event will be in place by September. El Niño conditions tend to decrease Atlantic hurricane activity, by increasing wind shear over the tropical Atlantic. Wind shear has been close to average over the tropical Atlantic since the beginning of hurricane season in June, though.

Figure 2. Arctic sea ice extent in 2012 (black line) compared to the previous record low year of 2007 (blue line) shows that 2012 is fast approaching all-time record territory. A big Arctic storm with a central pressure of 963 mb affected the ice during the first two weeks of August, causing a temporary downward spike in sea ice extent. Image credit: Danish Meteorological Institute.

Arctic sea ice falls to 2nd lowest extent in July, nears all-time record low during AugustJuly 2012 Arctic sea ice extent reached its 2nd lowest July extent in the 35-year satellite record, according to the National Snow and Ice Data Center (NSIDC). During the first half of August, sea ice has undergone a spectacular decline, and we are on pace to break the all-time lowest sea ice extent record set in September 2007. As of August 17, the University of Bremen was showing that sea ice extent has already broken the all-time record; the Danish Meteorological Institute put the ice loss in 2nd place behind September 2007; and the National Snow and Ice Data Center put Arctic ice loss in 3rd place behind September of 2007 and 2011.

Update on 94LA large tropical wave (Invest 94L) located a few hundred miles southwest of the Cape Verde Islands off the coast of Africa is headed west at 15 - 20 mph. This storm is a threat to develop into a tropical storm that will affect the Lesser Antilles Islands as early as Wednesday. The storm is under moderate wind shear of 10 - 20 knots, and is over waters of 28°C. A large area of dry air lies just to the north of 94L, as seen on the latest Saharan Air Layer (SAL) analysis. Satellite loops show that 94L has increased in organization this afternoon, with a growing amount of heavy thunderstorm activity and spin at middle levels of the atmosphere.

Figure 4. Afternoon satellite image of Invest 94L.

Forecast for 94LThe latest 2 pm EDT run of the SHIPS model predicts that wind shear will be low, 5 - 10 knots, and ocean temperatures will fluctuate around 28°C over the next five days, as 94L tracks westwards towards the Lesser Antilles. As is typical with storms making the crossing from Africa to the Antilles, dry air to the north will likely interfere with development, and the SHIPS model predicts increased dry air as 94L approaches the Lesser Antilles. However, with shear expected to be low, dry air may be less of an issue for 94L than it was for Ernesto or TD 7. The storm should maintain a nearly due west track through Monday night, to a point near 50°W, about 700 miles east of the Lesser Antilles. At that point, a trough of low pressure passing to the north of 94L may be able to pull the storm to the northwest well to the northeast of the Lesser Antilles, as suggested by the latest 12Z (8 am EDT) run of the NOGAPS model. The 12Z UKMET model shows a more west-northwesterly motion resulting in a near miss of the Lesser Antilles on Thursday. Our two best performing models--the GFS and ECMWF--have both been taking 94L through the Lesser Antilles with every run for the past 24 hours, though. The latest 12Z run of both models now agree on the timing, with 94L arriving Wednesday night or Thursday morning. The BAMM model, which performed as well as the ECMWF and GFS at 5-day forecasts in 2011, is also showing a track through the Lesser Antilles. Given this agreement among our top three models for long-range forecasts, I give a 60% chance that 94L will pass through the Lesser Antilles. In their 2 pm EDT Tropical Weather Outlook, NHC gave 94L a 50% chance of developing into a tropical depression by Monday afternoon.

94L a threat to the Lesser Antilles; Gordon a hurricane; Helene hits Mexico

A large tropical wave that emerged from the coast of Africa Thursday night (Invest 94L) is located a few hundred miles southwest of the Cape Verde Islands off the coast of Africa, and is headed west at 15 - 20 mph. This storm is a threat to develop into a tropical storm that will affect the Lesser Antilles Islands as early as Wednesday. The storm is under moderate wind shear of 10 - 20 knots, and is over waters of 27.5°C. A large area of dry air lies just to the north of 94L, as seen on the latest Saharan Air Layer (SAL) analysis. This morning's 8:15 am EDT ASCAT pass caught the east side of 94L, and showed a partial surface circulation. Satellite images show just a modest amount of heavy thunderstorm activity, and I expect the earliest that 94L could develop into a tropical depression would be Sunday.

Figure 1. Morning satellite image of Invest 94L.

Forecast for 94LThe latest 8 am EDT run of the SHIPS model predicts that wind shear will be low, 5 - 10 knots, and ocean temperatures will gradually warm from 27.5°C to 28.5°C over the next four days, as 94L tracks westwards towards the Lesser Antilles. As is typical with storms making the crossing from Africa to the Antilles, dry air to the north will likely interfere with development. However, with shear expected to be low, dry air may be less of an issue for 94L than it was for Ernesto or TD 7. The storm should maintain a nearly due west track through Monday night, to a point near 50°W, about 700 miles east of the Lesser Antilles. At that point, a trough of low pressure passing to the north of 94L may be able to pull the storm more to the northwest, as suggested by the latest 06Z (2 am EDT) run of the NOGAPS model, and by three members of the GFS model ensemble forecast (Figure 2.) However, the models have been trending more towards a solution where this trough is not strong enough to influence 94L's path. This scenario will be more likely if 94L takes its time to develop, since a weaker storm will be smaller and shallower, and less likely to respond to the trough passing to the north. Our two best performing models, the GFS and ECMFW, both take 94L through the Lesser Antilles. The ECMWF, which predicts that 94L will stay weak and not develop, is faster, bringing the storm through the Lesser Antilles on Wednesday. The GFS model is slower, bringing 94L to the Lesser Antilles on Thursday as a hurricane. The models have shown poor run-to-run consistency in both the timing and the track of 94L, so it is difficult to assess which land areas might be most at risk, and when. A database of historical probabilities of storms in the same location as 94L maintained by Dr. Bob Hart of Florida State University reveals that historically, 45% of storms in this location have eventually hit land, with Canada (13% chance) and North Carolina (15% chance) the most likely targets. In their 8 am EDT Tropical Weather Outlook, NHC gave 94L a 40% chance of developing into a tropical depression by Monday morning.

Figure 2. The 00Z (8 pm EDT) run of the GFS model from August 17, 2012, was done 20 different times at low resolution using slightly different initial conditions to generate an ensemble of forecasts (pink lines.) The high-resolution operational GFS forecast is shown in white.

Gordon becomes a hurricaneHurricane warnings are flying for the central and eastern Azores Islands as Hurricane Gordon heads eastwards at 18 mph. Gordon became the third hurricane of the 2012 Atlantic hurricane season at 5 am Saturday morning, and is sporting an impressive-looking eye on visible satellite loops. Gordon should be able to maintain hurricane status until Sunday, when wind shear will rise steeply to 30 - 40 knots, and ocean temperatures will drop to 25°C. The combined effects of high wind shear, dry air, and cooler waters will likely act to weaken Gordon to a strong tropical storm by the time it arrives in the Azores Islands Sunday night, but the storm will be strong enough to bring damaging winds and heavy rain to the Azores Islands. Gordon is not a threat to any other land areas, and the extratropical remnants of Gordon will not bring any strong winds or significant rain to Europe. The last time the Azores were affected by a tropical storm was in 2009, when Tropical Storm Grace brought 65 mph winds on October 4. No significant damage was reported. Ironically, the last hurricane to affect the Azores was the 2006 version of Hurricane Gordon, which caused minor damage in the Azores, consisting of mostly fallen trees and power outages. However, after Gordon became an extratropical low, four injuries due to falling debris from high wind were reported in Spain, and Gordon brought high winds and rain that affected practice rounds at the Ryder Cup golf tournament in Ireland. About 126,000 homes were without power after the storm in Northern Ireland and one injury was reported.

Figure 3. Morning satellite image of Hurricane Gordon.

Helene makes landfall in MexicoTropical Storm Helene made landfall near 10 am EDT as a tropical storm near Tampico, Mexico, with 40 mph winds. Helene's formation on August 17 ties 2012 with 1933 for the 2nd earliest appearance of the Atlantic's eighth tropical storm. Helene's rains should remain south of Texas, but moisture from Helene may feed into a stalled frontal system over the northern Gulf of Mexico and bring heavy rains to the northern Gulf Coast early next week.

I'll have a new post this afternoon, giving a quick global weather summary for July, the 4th hottest July in Earth's history.

After a long path across the Atlantic that mirrored the track of Hurricane Ernesto, Tropical Depression Seven finally got its act together enough over the Bay of Campeche tropical storm breeding grounds to earn the name Helene. Helene's formation on August 17 ties 2012 with 1933 for the 2nd earliest appearance of the Atlantic's eighth tropical storm. Only 2005 had an earlier formation date of the season's eighth storm. Most of this year's storms have been weak, though, so the total Accumulated Cyclone Energy (ACE) of 21 is not that much higher than the normal year-to-date ACE of 15, based on a 1981 - 2010 ACE climatology. The normal yearly ACE for the Atlantic is 104. Helene doesn't have much room to work with before landfall, but has the potential to be a prodigious rainmaker for Mexico, with NHC predicting 5 - 10 inches for portions of Northeast Mexico. This part of the coast is not in drought, so will be prone to heavy flooding. Fortunately, Ernesto's main rains fell to the south of where Helene's rains are falling. Helene's rains should remain south of Texas, though we can't rule out a few thunderstorms bringing 1 - 2 inches of rain to extreme South Texas on Saturday and Sunday.

Figure 1. Afternoon satellite image of Helene.

94L a potential threat to the Lesser AntillesA large tropical wave emerged from the coast of Africa Thursday night, and was designated Invest 94L by NHC this Friday morning. The models have been impressed this system, and develop it into a tropical storm by the middle of next week. The latest set of 12Z (8 am EDT) model runs are divided on how far west 94L will make it before curving more to the northwest. The ECMWF model keeps 94L weak for the next 5 - 6 days, and has progressively been bringing the storm closer to the Lesser Antilles Islands with each successive run. The ECMWF predicts 94L will pass very close to the northern Lesser Antilles August 24 - 25 as a weak tropical storm. The 12Z GFS model predicted recurvature of 94L well to the east of the Lesser Antilles and Bermuda, but the latest 18Z run has the storm plowing through the Lesser Antilles on Thursday, August 23, as a strong tropical storm, then becoming a hurricane in the Eastern Caribbean. Given that our two top models for forecasting hurricane tracks are increasingly showing a threat to the Lesser Antilles, residents of the islands should pay close attention to the progress of 94L. The eventual track of 94L will depend on the strength of the storm over the next seven days, which is difficult to forecast, since 94L will have the usual trouble with dry air to the north from the Saharan Air Layer (SAL.) With the models changing their tune drastically from run to run, its tough to say what land areas might be most at risk from the storm in the long term. In their 2 pm EDT Tropical Weather Outlook, NHC gave 94L a 10% chance of developing into a tropical depression by Sunday afternoon.

Satellite loops show that a small area of disturbed weather with an increasing amount of heavy thunderstorms has developed in southern Gulf of Mexico's Bay of Campeche, in association with the remnants of Tropical Depression Seven. Heavy rains from ex-TD-7 are beginning to impact the coast of Mexico between Tampico and Veracruz, and radar from Mexico shows some rotation to the echoes, but little in the way of spiral banding. With wind shear a light 5 -10 knots and very warm ocean waters of 30°C to take advantage of, ex-TD-7 has a 70% chance of becoming a tropical cyclone before it makes landfall over the Mexican coast on Saturday, said NHC in their 8 am EDT Tropical Weather Outlook. A hurricane hunter aircraft will investigate ex-TD-7 this afternoon around 2 pm EDT. Ex-TD-7's west-northwest to northwest motion should continue through landfall, and the storm may be capable of bringing heavy rains of 3 - 6 inches to the Mexican coast near Tampico. Brownsville, Texas should stay just north of the heavy rain area of ex-TD-7.

Figure 1. Morning satellite image of ex-TD 7.

Tropical Storm Gordon heads toward the AzoresTropical Storm Gordon continues eastwards towards the Azores, and is not a threat to any other land areas. Satellite loops show Gordon has a respectable amount of organization and heavy thunderstorm activity. Gordon's environment has gotten marginal for a hurricane--wind shear is moderate, 10 - 20 knots, and ocean temperatures are near 26.5°C, which is right at the border of where hurricanes can usually exist. Water vapor satellite loops show a large region of dry air on the south side of the storm. The 8 am EDT run of the SHIPS model predicts that wind shear will remain moderate through Saturday night, then rise steeply to 30 - 40 knots on Sunday. At the same time, ocean temperatures will drop to 25°C. By Sunday, the combined effects of high wind shear, dry air, and cooler waters will likely act to weaken Gordon and make it no longer tropical. However, Gordon will probably still be strong enough Sunday night to potentially bring damaging winds and heavy rain to the Azores Islands. The extratropical remnants of Gordon will not bring any strong winds or significant rain to Europe.

Figure 2. Tropical Storm Gordon as seen by NASA's Terra satellite at 10:25 am EDT August 16, 2012. At the time, Gordon was strengthening, with 50 mph top winds. Image credit: NASA.

94L developing off the coast of AfricaA large tropical wave emerged from the coast of Africa Thursday night, and was designated Invest 94L by NHC this Friday morning. The models have been very gung-ho on this system, and develop it into a tropical storm by the middle of next week. 94L will follow a west to west-northwest track over the next week, and may threaten the northern Lesser Antilles Islands as early as Friday, August 24. This storm could eventually affect Bermuda, the U.S. East Coast, or Canada 10 - 14 days from now, but could also recurve harmlessly out to sea well before reaching the Lesser Antilles, as suggested by the 00 UTC run of the GFS model. In their 8 am EDT Tropical Weather Outlook, NHC gave 94L a 10% chance of developing into a tropical depression by Sunday morning.

Comparing the 2012 drought to the Dust Bowl droughts of the 1930s

The great U.S. drought of 2012 remained about the same size and intensity over the past week, said NOAA in their weekly U.S. Drought Monitor report issued Thursday, August 16. The area of the contiguous U.S. covered by drought remained constant at 62%, and the area covered by severe or greater drought also remained constant at 46%. However, the area covered by the highest level of drought--exceptional--increased by 50%, from 4% to 6%. Large expansions of exceptional drought occurred over the heart of America's grain producing areas, in Kansas, Nebraska, Oklahoma, and Missouri. The new NOAA State of the Climate Drought report for July 2012 shows that the 2012 drought is 5th greatest in U.S. history, and the worst in 56 years. The top five years for area of the contiguous U.S. covered by moderate or greater drought:

Figure 1. August 14, 2012 drought conditions showed historic levels of drought across the U.S., with 62% of the contiguous U.S. experiencing moderate or greater drought, and 46% of the county experiencing severe or greater drought. Image credit: U.S. Drought Monitor.

Comparison with the great Dust Bowl droughts of the 1930sAn important fact to remember is that the 2012 drought is--so far--only a one-year drought. Recall that 2011 saw record rains that led to unprecedented flooding on the Mississippi, Ohio, and Missouri Rivers. In contrast, the great droughts of the 1950s and 1930s were multi-year droughts. The Dust Bowl drought of the 1930s lasted up to eight years in some places, with the peak years being 1934, 1936, and 1939 - 1940. Once the deep soil dries out, it maintains a memory of past drought years. This makes is easier to have a string of severe drought years. Since the deep soil this summer still maintains the memory of the very wet year of 2011, the 2012 drought will be easier to break than the Dust Bowl droughts of the 1930s were.

During the 1920s, agriculture in the United States expanded into the central Great Plains. Much of the original, drought-resistant prairie grass was replaced with drought-sensitive wheat. With no drought plan and few erosion-control measures in place, this led to large-scale crop failures at the initiation of the drought, leaving fields devegetated and barren, exposing easily eroded soil to the winds. This was the source of the major dust storms and atmospheric dust loading of the period on a level unprecedented in the historical record.

Figure 2. Black Sunday: On April 14, 1935 a "Black Blizzard" hit Oklahoma and Texas with 60 mph winds, sweeping up topsoil loosened by the great Dust Bowl drought that began in 1934.

The Dust Bowl drought and heat of the 1930s: partially human-causedUsing computer models of the climate, the scientists found that the Dust Bowl drought was primarily caused by below-average ocean temperatures in the tropical Pacific and warmer than average ocean temperatures in the Atlantic, which acted together to alter the path of the jet stream and bring fewer precipitation-bearing storms to the Central U.S. However, the full intensity of the drought and its spatial extent could not be explained by ocean temperature patterns alone. Only when their model included the impact of losing huge amounts of vegetation in the Plains due to poor farming practices could the full warmth of the 1930s be simulated. In addition, only by including the impact of the dust kicked up by the great dust storms of the Dust Bowl, which blocked sunlight and created high pressure zones of sinking air that discouraged precipitation, could the very low levels of precipitation be explained. The Dust Bowl drought had natural roots, but human-caused effects made the drought worse and longer-lasting. The fact that we are experiencing a drought in 2012 comparable to the great Dust Bowl drought of the 1930s--without poor farming practices being partially to blame--bodes ill for the future of drought in the U.S. With human-caused global warming expected to greatly increase the intensity and frequency of great droughts like the 2012 drought in coming decades, we can expect drought to cause an increasing amount of damage and economic hardship for the U.S. Since the U.S. is the world's largest food exporter, this will also create an increasing amount of hardship and unrest in developing countries that rely on food imports.

Tropical Storm Gordon is here, born out of a tropical wave that emerged from the coast of Africa last week. Gordon's formation puts the hurricane season of 2012 in fifth place for the earliest date of formation of the season's seventh storm, going back to 1851. Only 2005, 1936, 2011, and 1995 had earlier formation dates of the season's seventh storm. Satellite loops show Gordon has developed a Central Dense Overcast (CDO)--a large and expanding area of high cirrus clouds over the center, due to a build-up of heavy thunderstorms. This is characteristic of intensifying tropical storms. Wind shear is light, but ocean temperatures are on the cool side, near 27°C. Water vapor satellite loops show that Gordon has moistened its environment considerably, but a large region of dry air lurks on three sides of the storm, ready to barge in and disrupt Gordon when wind shear rises on Saturday. The 8 am EDT run of the SHIPS model predicts that wind shear will remain light to moderate through Friday, then rise steeply to 25 - 40 knots over the weekend. At the same time, ocean temperatures will drop to 26°C. By Sunday, the combined effects of high wind shear, dry air, and cooler waters will likely act to weaken Gordon and make it no longer tropical, but Gordon will probably still be strong enough Sunday night to potentially bring damaging winds and heavy rain to the Azores Islands.

Figure 1. Morning satellite image of Tropical Storm Gordon.

Gordon's place in historyThe 2012 version of Gordon is the fourth storm that has been given that name. Previous incarnations of Gordon appeared in 1994, 2000, and 2006. It's pretty unlikely that the 2012 version of Gordon will get its name retired, but the name Gordon should have been retired long ago. During the first appearance of Gordon in November 1994, the storm moved very slowly over Eastern Cuba, and dropped prodigious rains over Haiti. The resulting flash flooding killed over 1,100 people. Unquestionably, the 1994 version of Gordon should have had its name retired, due to the devastating impact it had on Haiti. However, after the 1994 hurricane season, Haiti did not send a representative to the annual World Meteorological Organization meeting that decides retirement of hurricane names, and no other country affected by Gordon requested that the name be retired.

Figure 2. Track of the 1994 version of Hurricane Gordon. The storm killed over 1100 people in Haiti.

Elsewhere in the tropicsIn the Gulf of Mexico, a fall-like cold front is expected to stall out early next week. Wind shear is predicted to be low to moderate, and cold fronts stalled out over the Gulf of Mexico often serve as the seed for tropical storms. The most likely formation location of such a storm would be off the Texas coast, or off the Mexican coast south of Texas.

A large tropical wave is emerging from the coast of Africa today, and the GFS and ECMWF models predict this wave will develop into a tropical depression 4 - 5 days from now. Preliminary indications are that this new storm will follow a path similar to Gordon's, recurving to the east of Bermuda, but it is too early to be confident of this.

Hottest rain on record? Rain falls at 115°F in Needles, California

A searing heat wave rare even for the Desert Southwest sent temperatures soaring to record levels on Monday, with Needles, California tying its record high for the date of 118°F (47.8°C). The temperature might have gone higher in Needles, but a thunderstorm rolled in at 3:20 pm, and by 3:56 pm PDT, rain began falling at a temperature of 115°F (46.1°C). Most of the rain evaporated, since the humidity was only 11%, and only a trace of precipitation was recorded in the rain gauge. Nevertheless, Monday's rain at 115° in Needles sets a new world record for the hottest rain in world history. I don't think many people were outside to experience to experience the feeling of rain falling at 115°, but if they were, it must have been an uncomfortable, sauna-like experience! Thanks go to Dr. Warren Blier of the NWS Monterey office for pointing out this remarkable event to me.

It is exceedingly rare to get rain when the temperature rises above 100°F, since those kind of temperatures usually require a high pressure system with sinking air that discourages rainfall. Monday's rain in Needles was due to a flow of moisture coming from the south caused by the Southwest U.S. monsoon, a seasonal influx of moisture caused by the difference in temperature between the hot desert and the cooler ocean areas surrounding Mexico to the south. According to weather records researcher Maximiliano Herrera, the previous record for hottest rain, which I blogged about in June, was a rain shower at 109°F (43°C) observed in Mecca, Saudi Arabia on June 5, 2012 and in Marrakech, Morocco on July 10, 2010. The 11% humidity that accompanied Monday's rain shower at 115° in Needles was the lowest humidity rain has ever occurred at anywhere on Earth in recorded history, according to Mr. Herrera.

Figure 1. True-color MODIS satellite image of California and Arizona taken at 1:25 pm PDT August 13, 2012. Developing thunderstorms surround Needles, CA, and the line of clouds to the southwest of the city would develop into a thunderstorm that brought rain to the city at 4 pm PDT, at a temperature of 115°F. Image credit: NASA.

A "very rare" heat wave for PhoenixThe heat wave that brought Needles' record hot rain has broken an exceptional number of heat records in Phoenix, Arizona the past two weeks. According to the Phoenix NWS office, the "almost unbearable heat" of the first two weeks of August is a "very rare" event, and August 1 - 14, 2012 was the warmest such 2-week period in city history. The average temperature on August 6 - 13 was 100°F or higher each of the eight days, tying the record for most consecutive days with an average temperature of 100°. The temperature peaked at 116° on August 8, just 6° below Phoenix's all-time record of 122° set on June 26, 1990. The forecast for Phoenix call for a bit of relief--highs are expected to be a relatively modest 105° today, and down near 100° by Friday.

Figure 2. Morning satellite image of 93L over the Central Atlantic.

93L close to tropical depression statusA large tropical wave (Invest 93L) is located in the Central Atlantic about 700 miles east of Bermuda. Satellite loops this morning show a surface circulation has formed, and heavy thunderstorm activity has increased to the point where 93L should be considered a tropical depression, if the heavy thunderstorms can persist through this afternoon. Wind shear is light, and ocean temperatures are warm, near 28°C. The latest Saharan Air Layer Analysis from the University of Wisconsin shows that 93L has moistened its environment considerably, and dry air should no longer be a significant impediment to development. The 8 am EDT run of the SHIPS model predicts that wind shear will remain in the low range through the weekend, and I expect this system will become Tropical Storm Gordon by Friday. The storm will not affect Bermuda, but residents of the Azores Islands should keep an eye on 93L, which could pass through the islands as early as Sunday night. In their 8 am Tropical Weather Outlook, NHC gave 93L an 80% chance of becoming a tropical depression by Friday morning.

Elsewhere in the tropicsIn the Gulf of Mexico, a fall-like cold front is expected to stall out early next week, and the GFS model is predicting something could start to spin up near the Texas/Mexico border on Monday. Wind shear is predicted to be low to moderate, and cold fronts stalled out over the Gulf of Mexico often serve as the seed for tropical storms.

Most of the models predict development of a new tropical wave off the coast of Africa 6 - 7 days from now.

The odds of a hurricane spoiling the Republican National Convention in Tampa

On September 25, 1848, the Great Gale of 1848, the most violent hurricane in Tampa's history, roared ashore as a Category 3 or 4 hurricane with 115 - 135 mph winds. Major R. D. S. Wade weathered the storm in Fort Brooke, in what is now downtown Tampa. Here is what he wrote this to his commanding officer in Washington D.C.: "The waters rose to an unprecedented height, and the waves swept away the wharves and all the buildings that were near the Bay or river." A 15-foot storm surge was observed at Fort Brooke, and the peninsula where St. Petersburg lies in Pinellas County was inundated "at the waist" and "the bays met," making St. Petersburg an island. After the hurricane, "Tampa was a scene of devastation. Magnificent old oaks were toppled by the hurricane's winds. At Fort Brooke the barracks, horse shed, and other structures were gone. The pine forest north of the garrison was filled with wreckage and debris. The hurricane's powerful surge had shifted sand all along the coast and reshaped many of the keys near Tampa Bay. Navigation routes were filled in and closed, making charts of the area produced before 1848 almost useless after the hurricane. In terms of intensity and destruction, the 1848 storm remains perhaps the greatest in Tampa's history" (Barnes, 1999.)

Figure 1. Pencil sketch of the Captains' Quarters, drawn by one of the officers stationed at Fort Brooke in 1845. Fort Brooke was one of the largest military establishments in the United States at the time. Image Credit: The Tampa Bay History Center.

Fort Brooke todayFort Brooke is the current site of the Tampa Bay Convention Center, which hosts the Republican National Convention on August 27 - 30 this year. The convention center is in Evacuation Zone A, which is evacuated for Category 1 hurricanes. The Tampa Bay Times Forum and two major convention hotels--the Tampa Marriott Waterside and the Embassy Suites--are in Evacuation Zone B, which is evacuated for Category 2 hurricanes. In a worst-case Category 4 hurricane, the Convention Center could be immersed in 20 feet of water. Clearly, even a Category 1 hurricane would be enough to spoil the convention. So, what are the odds of a mass evacuation order being issued for Tampa Bay during the convention?

Figure 2. Predicted height above ground of the water from a worst-case Category 4 hurricane in the Tampa Bay region, as computed using NOAA's SLOSH storm surge model. The Tampa Bay convention center would go under 20 feet of water, and St. Petersburg would become an island, as occurred during the 1848 hurricane.

Figure 3. Perhaps the most spectacular hurricane image ever captured: view of Hurricane Elena on September 1, 1985, as seen from the Space Shuttle Discovery. At the time, Elena was a Category 3 hurricane with 125 mph winds, located just 80 miles offshore from Tampa Bay, Florida. The hurricane prompted the largest mass evacuation in Tampa Bay history.

Two mass evacuations in Tampa in the past 25 yearsTwo hurricanes have prompted mass evacuations of more than 300,000 people from the Tampa Bay area over the past 25 years. The first was Hurricane Elena of 1985, a Category 3 hurricane that stalled 80 miles offshore for two days on Labor Day weekend, bringing a 6 - 7 foot storm surge, wind gusts of 80 mph, and torrential rains. On August 13, 2004, another mass evacuation was ordered for Hurricane Charley. Thanks to a late track shift, Charley missed Tampa Bay, and instead hit well to the south in Port Charlotte as a Category 4 storm with 150 mph winds. More limited evacuations of low-lying areas and mobile homes in the 4-county Tampa Bay region were ordered for three other hurricanes in the past fifteen years--Hurricane Georges of 1998, Hurricane Frances of 2004, and Hurricane Jeanne of 2004. Other historical storms which would likely trigger mass evacuations were they occur today include:

The 1921 hurricane. One of only two major hurricanes to hit Tampa, this Category 3 storm brought a storm tide of 10.5 feet.

Hurricane Easy of 1950. The hurricane parked itself over the west coast of Florida, drenching residents with record-breaking rains, and brought a 6.5 ft storm surge to Tampa Bay.

Figure 4. Damage to Bayshore Boulevard after the 1921 Tampa Bay hurricane. The road leads to the Tampa Bay Convention Center from the south.

Figure 5. Track of the Tampa Bay Hurricane of 1921, one of only two major hurricanes ever to hit the city. This Category 3 storm with 115 mph winds brought a storm tide of 10.5 feet to Tampa Bay.

Figure 6. A near miss: just a slight deviation in the path of Hurricane Charley of 2004 would have brought the Category 4 hurricane into Tampa Bay.

Tampa Bay's vulnerability to hurricanesTampa Bay doesn't get hit very often by hurricanes. The last time it suffered a direct hit by any hurricane was 1946, when a Category 1 storm came up through the bay. The Tampa Bay Hurricane of October 25, 1921 was a the last major hurricane to make landfall in the Tampa Bay Region. At that time, there were 160,000 residents in the 4-county region, most of whom lived in communities on high ground. Today there are 2.75 million residents in the region, most of whom live along the coast and low-lying areas or in manufactured housing. About 1/3 of the 4-county Tampa Bay region lies within a flood plain. Over 800,000 people live in evacuation zones for a Category 1 hurricane, and 2 million people live in evacuation zones for a Category 5 hurricane, according to the 2010 Statewide Regional Evacuation Study for the Tampa Bay Region. Given that only 46% of the people in the evacuation zones for a Category 1 hurricane evacuated when Category 4 Hurricane Charley threatened the region, the potential for hundreds or thousands of people to die when the next major hurricane hits the region is high. In the long run, I expect a multi-billion dollar sea wall will be built to protect Tampa Bay from storm surges, since sea level rise will make storm surge damages increasingly problematic. A 2007 study by Tufts University titled, Florida and Climate Change, found that a 2.25 foot increase in sea level--which many sea level rise scientists expect will happen by the end of the century--would put 152,000 people in Pinellas County (where St. Petersburg is located) at risk of inundation.

The hurricane forecast for the Republican National ConventionGiven that there have been two mass evacuations of Tampa during the past 25 years during the peak three-month period of hurricane season--August, September, and October--history suggests that the odds of a mass evacuation order being given during the 4-day period that the Republican National Convention is in town are probably around 0.2%. Any tropical waves which might develop into hurricanes that could hit Tampa during the convention would have to come off the coast of Africa next week. Looking at the latest 16-day forecast from the GFS, all of the tropical waves coming off of Africa next week are predicted to exit too far north to make the long crossing of the Atlantic and threaten the Gulf Coast. While something could develop in the Gulf of Mexico from the remains of an old cold front, it is rare for such storms to grow strong enough to deserve mass evacuations. So far, early signs point to a hurricane-free Republican National Convention at the end of August.

ReferencesBarnes, J., 1999, Florida’s Hurricane History. The University of North Carolina Press.

93L a possible threat to the Azores

A large tropical wave (Invest 93L) is located in the Central Atlantic about 1000 miles southeast of Bermuda. Satellite loops this morning show a surface circulation attempting to form, and heavy thunderstorm activity has increased markedly since Monday. Wind shear is light, and ocean temperatures are warm, near 27.5°C. The latest Saharan Air Layer Analysis from the University of Wisconsin shows that 93L has moistened its environment considerably, but dry air is still a significant impediment to development. The 8 am EDT run of the SHIPS model predicts that mid-levels of the atmosphere will remain very dry through Wednesday, then moisten considerably. The storm is expected to move west-northwest and then north, recurving well to the east of Bermuda. By Saturday, as 93L is headed northeastwards towards the Azores Islands, the GFS and NOGPAS models predict development into a tropical depression. Residents of the Azores Islands should keep an eye on 93L, which could pass close to the islands as early as Sunday night. In their 8 am Tropical Weather Outlook, NHC gave 93L a 30% chance of becoming a tropical depression by Thursday morning. Given the recent increase in 93L's organization, I put these odds at 50%.

Figure 1. Morning satellite image of 93L over the Central Atlantic.

Ex-TD-7 not a threat to developThe remains of Tropical Depression Seven are headed westward at 20 mph across the Western Caribbean, and will bring heavy rain to Honduras and Nicaragua today. These heavy rains will spread to northern Guatemala, Belize, and Mexico's Yucatan Peninsula tonight. Although wind shear is low and ocean temperatures high, there is not going to be enough time for ex-TD 7 to develop before it moves inland over the Yucatan Peninsula. None of the reliable models forecasts that ex-TD 7 will regenerate, and in their 8 am Tropical Weather Outlook, NHC gave ex-TD 7 a 20% chance of becoming a tropical depression by Thursday morning.

Watch the Gulf of Mexico this weekendAnother area to watch this weekend will be the Gulf of Mexico, where a fall-like cold front is expected to stall out. Wind shear is predicted to be low to moderate this weekend, and cold fronts stalled out over the Gulf of Mexico often serve as the seed for tropical storms. The GFS model has been showing some development may occur in the waters close to shore near the Texas/Mexico border early next week.

I'll have a new post early this afternoon. What are the chances that a hurricane will force a mass evacuation of Tampa during the Republican National Convention, August 27 - 30?

Two minor threat areas to watch in the Atlantic

The remains of Tropical Depression Seven are headed westward at 20 mph across the Central Caribbean, and will bring heavy rain to Jamaica and Southwest Haiti today. Satellite loops show that ex-TD 7 has only a modest amount of heavy thunderstorms, with no signs of any organization. High wind shear of 15 - 25 knots should prevent the system from regenerating today. Wind shear will fall to the low range over the disturbance on Tuesday and Wednesday, but there is not going to be enough time for ex-TD 7 to develop before it moves inland over the Yucatan Peninsula on Wednesday. None of the reliable models forecasts that TD 7 will regenerate, and in their 8 am Tropical Weather Outlook, NHC gave ex-TD 7 a 10% chance of becoming a tropical depression by Wednesday morning.

Figure 1. Morning satellite image of 93L over the Central Atlantic.

93LA large tropical wave (Invest 93L) is located in the Central Atlantic about 1200 miles west-northwest of the Cape Verde Islands. Satellite loops show a pronounced spin to the atmosphere at mid levels, but little in the way of heavy thunderstorm activity. The latest Saharan Air Layer Analysis from the University of Wisconsin shows that 93L is surrounded by a large area of dry air from the Sahara, which has infiltrated into the storm. This dry air will be a significant impediment to development, as the 8 am EDT run of the SHIPS model predicts that mid-levels of the atmosphere will remain very dry during the next four days. The storm is expected to move west-northwest and then north, recurving well to the east of Bermuda. By Saturday, as 93L is headed northeastwards towards the Azores Islands, the disturbance will find a moister environment, and the GFS model predicts development into a tropical storm will occur. None of the other reliable models predict development, but residents of the Azores Islands should keep an eye on 93L. In their 8 am Tropical Weather Outlook, NHC gave 93L a 10% chance of becoming a tropical depression by Wednesday morning.

Another area to watch this weekend will be the Gulf of Mexico, where a fall-like cold front is expected to stall out. Though no models are showing tropical storm development will occur, wind shear is predicted to be low to moderate this weekend, and cold fronts stalled out over the Gulf of Mexico often serve as the seed for tropical storms.

Remains of TD 7 kill two in Trinidad; 93L too dry to develop

The remains of Tropical Depression Seven brought heavy rains to Trinidad on Saturday, unleashing flooding and mudslides that killed two people and left two others missing in Diego Martin. The storm is headed westward at 20 mph across the Caribbean, but high wind shear of 15 - 25 knots should prevent the system from regenerating. None of the reliable models forecasts that TD 7 will regenerate, and the storm should arrive in Mexico's Yucatan Peninsula on Wednesday. In their 8 am Tropical Weather Outlook, NHC gave ex-TD 7 a 10% chance of becoming a tropical depression by Tuesday morning.

93LA large tropical wave(Invest 93L) is located in the Central Atlantic about 700 miles west-northwest of the Cape Verde Islands. Satellite loops show a pronounced spin to the atmosphere at mid levels, but little in the way of heavy thunderstorm activity. The latest Saharan Air Layer Analysis from the University of Wisconsin shows that 93L is surrounded by a large area of dry air from the Sahara, which has infiltrated into the storm. This dry air will be a significant impediment to development, as the 8 am EDT run of the SHIPS model predicts that mid-levels of the atmosphere will remain very dry during the next four days. None of the reliable computer models develop 93L during the coming five days, and the storm is expected to move west-northwest and then north, recurving well to the east of Bermuda. In their 8 am Tropical Weather Outlook, NHC gave 93L a 10% chance of becoming a tropical depression by Tuesday morning. The GFS model is predicting 93L will develop into a tropical depression in about a week, once the storm recurves and begins heading northeast. At this point, it does not appear 93L will trouble any land areas.

TD 7 dies; 93L little threat to develop

Tropical Depression Seven is dead, ripped apart by moderate wind shear of 10 - 20 knots this Saturday morning. The storm was too small and fragile to survive the shear, thanks to dry air from the Sahara that had infiltrated the storm environment. None of the reliable models forecasts that TD 7 will regenerate as it tracks westwards across the Caribbean over the next four days. The remains of the storm should arrive in Mexico's Yucatan Peninsula on Wednesday.

Figure 1. Morning satellite image of the remnants of TD 7.

93LA large tropical wave(Invest 93L) is located in the Eastern Atlantic about 300 miles northwest of the Cape Verde Islands. Satellite loops show a pronounced spin to the atmosphere at mid levels, but little in the way of heavy thunderstorm activity. The latest Saharan Air Layer Analysis from the University of Wisconsin shows that there is a large area of dry air from the Sahara surrounding 93L, which has infiltrated into the storm. This dry air will be a significant impediment to development during the coming week, as the 8 am EDT run of the SHIPS model predicts dryness at mid-levels of the atmosphere will increase during the next five days. None of the reliable computer models develop 93L during the coming seven days, and the storm is expected to move west-northwest and then north, recurving well to the east of Bermuda. In their 8 am Tropical Weather Outlook, NHC gave 93L a 20% chance of becoming a tropical depression by Monday morning.

July 2012: hottest month in U.S. history

During the 1930s, a series of epic heat waves gripped the U.S., drying up the soil in an unprecedented drought that brought about the great Dust Bowl. The most intense heat hit during July 1936, which set a record for hottest month in U.S. history that stood for 76 years. That iconic record has now fallen, bested by 0.2°F during July 2012, which is now the hottest month in U.S. history, said NOAA's National Climatic Data Center (NCDC) this week. So far in 2012, we've had the warmest March on record, 3rd warmest April, 2nd warmest May, and warmest July. These remarkably warm months have helped push temperatures in the contiguous U.S. to the warmest on record for the year-to-date period of January - July, and for the 12-month period August 2011 - July 2012. Twenty-four states were record warm for that 12-month period, and an additional twenty states were top-ten warm. The past fourteen months have featured America's 2nd warmest summer (in 2011), 4th warmest winter, and warmest spring. The summer of 2012 is on pace to be a top-five warmest summer on record, and could beat the summer of 1936 as the warmest summer in U.S. history.

Figure 1. When the temperature peaked at an all-time high of 108° in Minneapolis, Minnesota on July 14, 1936, the want-ad staff at the 'St. Paul Daily News' was provided with 400 pounds of ice and two electric fans to cool the air in the press room. Photo from the Minnesota Historical Society.

Figure 2. Year-to-date temperature, by month, for 2012 (red), compared to the other 117 years on record for the contiguous U.S., with the five ultimately warmest years (orange) and five ultimately coolest years (blue) noted. The 2012 data are still preliminary. The year-to-date period of January - July was the warmest on record by a huge margin--1.0°F. Image credit: NOAA/NCDC.

Figure 3. For the fourth consecutive month, a new U.S. record for hottest 12-month period was set in July 2012. Five of the top-ten warmest 12-month periods in the contiguous U.S. since 1895 have occurred since April 2011. Image credit: NOAA/NCDC.

Most extreme January - July period on recordNOAA's U.S. Climate Extremes Index (CEI), which tracks the percentage area of the contiguous U.S. experiencing top-10% and bottom-10% extremes in temperature, precipitation, and drought, was 46% during the year-to-date January - July period. This is the highest value since CEI record-keeping began in 1910, and more than twice the average value. Remarkably, 83% of the contiguous U.S. had maximum temperatures that were in the warmest 10% historically during the first seven months of 2012, and 74% of the U.S. of the U.S. had warm minimum temperatures in the top 10%. The percentage area of the U.S. experiencing top-10% drought conditions was 20%, which was the 16th greatest since 1910. Extremes in 1-day heavy precipitation events were the 24th greatest in the 103-year record.

Figure 4. NOAA's U.S. Climate Extremes Index (CEI) for January - July shows that 2012 has had the most extreme first seven months of the year on record, with 46% of the contiguous U.S. experiencing top-10% extreme weather.

Little change to the U.S. drought during the past weekThe great U.S. drought of 2012 remained about the same size and intensity over the past week, said NOAA in their weekly U.S. Drought Monitor report issued Thursday, August 9. The area of the contiguous U.S. covered by drought dropped slightly, from 63% to 62%, and the area covered by severe or greater drought stayed constant at 46%. The area of the country in the worst drought categories (extreme to exceptional drought) doubled from 10 percent last month to 22 percent this month. The extreme dryness and excessive heat devastated crops and livestock from the Great Plains to Midwest. During July, the area of the U.S. covered by moderate or greater drought was 57%, ranking as the 5th largest drought in U.S. history:

Video 1. This is Not Cool: Peter Sinclair's July 2012 video from the Yale Forum on Climate Change and the Media couples historical footage with contemporary clips and news segments. In one of the latter, for instance, NBC anchor Brian Williams opens the network’s flagship news program with the words: “It’s now official. We are living in one of the worst droughts of the past 100 years.” NASA scientist James Hansen is featured testifying about risks of “extreme droughts” in the nation’s breadbasket, and I'm featured in a few clips talking about the drought of 2012.

Little change to TD 7; 93L may develop off the coast of Africa

Tropical Depression Seven continues westward across the central tropical Atlantic with little change in appearance. Looking remarkably like the tropical depression that became Tropical Storm Ernesto last week, TD 7 has a limited region of heavy thunderstorms, as seen on satellite loops. A large amount of dry air to the west and north is visible on water vapor satellite loops, and this dry air is interfering with TD 7's heavy thunderstorms. Wind shear is a light 5 - 10 knots, which is favorable for intensification. Ocean temperatures are 26.5°C, which is a bit on the cool side, but these temperatures are 0.5° warmer than on Thursday. Winds at the Middle Atlantic buoy 41041 peaked at 29 mph, gusting to 38 mph, at 1:50 am EDT this Friday morning, when the center of TD 7 passed about 40 miles to the south. TD 7 will pass about 60 miles south of buoy 41040 near midnight tonight. The first hurricane hunter mission into TD 7 is scheduled for Saturday morning.

Figure 1. Morning satellite image of TD 7.

Forecast for TD 7TD 7 formed about 400 miles east-northeast of where Ernesto became a tropical depression, as seen on our wundermap. Atmospheric and oceanic conditions are currently quite similar to what Ernesto experienced, and I expect TD 7 will struggle with dry air like Ernesto did. The SHIPS model predicts that shear will remain light through tonight, then rise to the moderate range on Saturday, when the storm will encounter upper-level southwesterly winds associated with a trough of low pressure. However, ocean temperatures will warm to 28°C, which may partially counteract the increase in shear, as far as maintaining a favorable environment for development. Dry air and shear may be significant enough to destroy TD 7 on Sunday, as predicted by the GFS and ECMWF models. These models tried to kill off Ernesto in a similar situation last week, so I am inclined to believe TD 7 will survive for the coming five days, but struggle. The official NHC forecast of a 45 mph tropical storm moving through the Lesser Antilles Islands Saturday night and Sunday is a reasonable one.

Figure 2. Morning satellite image of Invest 93L.

93LA strong tropical wave in the far Eastern Atlantic that emerged off the coast of Africa Thursday night was designated Invest 93L by NHC. In their 8 am Tropical Weather Outlook, NHC gave 93L a 50% chance of becoming a tropical depression by Sunday morning. Satellite loops show a pronounced spin to the atmosphere at mid levels, but little in the way of heavy thunderstorm activity. The latest Saharan Air Layer Analysis from the University of Wisconsin shows that while the atmosphere immediately surrounding the disturbance is moist, there is a large area of dry air from the Sahara to the west and north. This dry air will be a significant impediment to development during the coming week. The ECMWF model shows some weak development of 93L over the coming week, and predicts a general west-northwesterly track. The storm may be something Bermuda needs to be concerned about in eight or so days.

Tropical Depression Seven forms

Tropical Depression Seven is here. Looking remarkably like the tropical depression that became Tropical Storm Ernesto on August 1, TD 7 has a limited region of heavy thunderstorms, as seen on satellite loops. A large amount of dry air to the west and north is visible on water vapor satellite loops, and this dry air is interfering with TD 7's heavy thunderstorms. Wind shear is a light 5 - 10 knots, which is quite favorable for intensification. Ocean temperatures are 26°C, which is a bit on the cool side, though. Winds at the Middle Atlantic buoy 41041 were 20 mph at 5 pm EDT, and should rise this evening, as the center of TD 7 passes about 50 miles to the south near midnight. The first hurricane hunter mission into TD 7 is scheduled for Saturday morning.

Figure 1. Satellite image of TD 7 as it was forming at 3:17 pm EDT August 9, 2012.

Forecast for TD 7TD 7 formed about 400 miles east-northeast of where Ernesto became a tropical depression, as seen on our wundermap. Atmospheric and oceanic conditions are currently quite similar to what Ernesto experienced, and I expect TD 7 will struggle with dry air like Ernesto did. The SHIPS model predicts that shear will remain light through Friday then rise to the moderate range on Saturday, when the storm will encounter moderate shear from upper-level westerly winds associated with a trough of low pressure. Since conditions are so similar to what Ernesto experienced, the official NHC forecast of a 45 - 50 mph tropical storm moving through the Lesser Antilles Islands Saturday and Sunday is a reasonable one.

92L near tropical depression status; Ernesto drenching Mexico

A tropical wave midway between Africa and the Lesser Antilles Islands (Invest 92L) has become well-organized, with satellite loops showing that a well-defined surface circulation formed around 9 am EDT. So far, 92L's heavy thunderstorm activity is limited, due to a large amount of dry air to the west and north that can be seen on water vapor satellite loops. However, heavy thunderstorms have recently increased near the new circulation center, and if current trends continue, 92L will likely be named Tropical Depression Seven later today. Wind shear is a light 5 - 10 knots, and the SHIPS model predicts that shear will remain light until Saturday morning, when the storm will encounter higher shear from upper-level westerly winds associated with a trough of low pressure. Of the six major computer models used operationally by NHC, only the NOGAPS model develops 92L, and not until Tuesday. The NOGAPS model did the best job of forecasting the genesis of Ernesto, though. There are some major timing differences between the models on how fast 92L will move. The GFS model predicts 92L will reach the Lesser Antilles Islands on Saturday, the ECMWF model has this happening on Sunday, and the NOGAPS model brings 92L though the northern Lesser Antilles on Monday. In their 8 am Tropical Weather Outlook, NHC gave 92L a 70% of developing into a tropical cyclone by Saturday morning. Residents of the Lesser Antilles should anticipate the possibility of another Ernesto-like situation, with tropical storm conditions affecting the islands Saturday through Monday.

Figure 1. Morning satellite image of Invest 92L.

ErnestoTropical Storm Ernesto is performing a tightrope act along the extreme southern edge of Mexico's Bay of Campeche, as the storm heads westwards towards its eventual doom over the mountains of Mexico. Ernesto made landfall Tuesday night at 11 pm EDT just north of the Belize/Mexico border as a Category 1 hurricane with 85 mph winds, weakened to a tropical storm with 50 mph winds while passing over the Yucatan Peninsula on Wednesday, but had enough time over water this morning to regenerate to a 70 mph tropical storm. Ernesto has a few more hours today when its center will be over water, and the storm's heavy rains of up to ten inches will cause flash flooding in Mexico's Veracruz state. So far, damage from Ernesto has been modest, with no deaths or injuries reported.

Figure 2. True-color MODIS image from NASA's Terra satellite of Tropical Storm Ernesto, taken at 1 pm EDT August 8, 2012. At the time, Ernesto had top winds of 50 mph. Image credit: NASA.

I'll have a new post this afternoon to talk about July 2012--the warmest month in U.S. history.

Ernesto hits the Yucatan with 85 mph winds

Ernesto is tropical storm again, after making landfall Tuesday night at 11 pm EDT just north of the Belize/Mexico border as a Category 1 hurricane with 85 mph winds. We have few good weather observations near the landfall location, though an automated weather station on Banco Chinchorro Island just off the coast of Mexico reported a minimum pressure of 979.4 mb. A personal weather station at the Margarita del Sol Costa Maya Resort recorded sustained winds of 42 mph, gusting to 49 mph, and a pressure of 988 mb in the west eyewall of Ernesto before the station failed. No other weather stations were in the eyewall, and we did not have a hurricane hunter aircraft in the storm at landfall to measure the winds, due to technical issues. Belize radar shows that Ernesto's eyewall has collapsed, but the storm has remained well-organized during its passage over the Yucatan Peninsula. Infrared satellite loops show that Ernesto's heavy thunderstorms are mainly affecting Mexico.

Figure 2. True-color MODIS image from NASA's Terra satellite of Tropical Storm Ernesto, taken at 12:15 pm EDT August 7, 2012. At the time, Ernesto had top winds of 65 mph. Image credit: NASA.

Forecast for ErnestoBoth radar and satellite loops show that Ernesto had a south of due west (260°) motion this morning, and it is no longer clear if the storm will re-emerge over the ocean on the west side of the Yucatan Peninsula. Even if Ernesto does move out over water again, it will be very close to the coast, limiting intensification potential. The official NHC forecast still calls for Ernesto to attain Category 1 hurricane strength over the Bay of Campeche. Ernesto's main threat will be heavy rainfall threat in the mountainous regions along its path through Mexico.

92LA tropical wave in the Eastern Atlantic (Invest 92L) is disorganized, with satellite loops showing limited heavy thunderstorm activity and a modest amount of spin at mid-levels of the atmosphere. A large amount of dry air to the west and north of 92L is interfering with development, as seen on water vapor satellite loops. Wind shear is a moderate 10 - 20 knots. None of the reliable computer models develop 92L. The GFS model predicts 92L will reach the Lesser Antilles Islands on Saturday, but the other major models show a much slower motion with no threat to the islands for at least six days. In their 8 am Tropical Weather Outlook, NHC gave 92L a 30% of developing into a tropical cyclone by Friday morning.

A tropical wave predicted to move off the coast of Africa on Thursday night is predicted to develop by both the GFS and ECMWF models.

Ernesto reaches hurricane strength as it approaches the Yucatan

Ernesto strengthened to a category 1 hurricane this afternoon after the morning hurricane hunter mission found winds of 80 mph, which is unusual for a hurricane without an eye wall. The hunters also found a minimum central pressure of 984 mb, which has dropped since this morning. The next hurricane hunter mission is scheduled to reach the center of the hurricane around 8pm EDT. The rain bands from Ernesto have reached the coast of Belize and Mexico as it continues to move west along the coast of Honduras, and landfall is expected north of the the Belize/Mexico border tonight around midnight. The Yucatan Basin buoy is now reporting gusts up to 53 mph, with sustained winds around 40 mph and 19 foot waves. These gusts are about 10 mph stronger than this morning's readings. Weather stations along the coast of Mexico and Belize aren't reporting winds stronger than 10 mph, however, they are expected to pick up around 8 or 9pm EDT tonight. The island of Roatan in Honduras is experiencing winds around 15 mph this afternoon, along with some light to moderate rainfall. Honduras seems to be the most impacted country so far, although they have avoided issuing evacuations. Nicaragua, however, has evacuated 1,500 people as of last night, and Mexico's authorities have evacuated around 600 residents from Punta Allen, which is a fishing village between Cozumel and Chetumal.

Visible satellite imagery suggests Ernesto still has the potential to develop an eye wall before landfall, as strong, organized thunderstorms are present in all four quadrants of the hurricane. Infrared satellite imagery shows the clockwise circulation at high levels (the upper level anti-cyclone) which will help ventilate the hurricane and could support further enhancement. If Ernesto wasn't approaching landfall, it would likely continue to strengthen and could have even experienced a period of rapid intensification, given the heat content of the Caribbean Sea. Wind shear around the hurricane remains low at 5-10 knots.

Figure 1. Radar image from Belize as the outer rain bands of Ernesto approach. This image was captured at 2:30pm EDT.

Forecast for ErnestoErnesto will continue to track west this afternoon and evening, making landfall north of the Belize/Mexico border around midnight tonight. Given the current state of the hurricane, some more intensification is possible over the next few hours as it approaches land. Heavy rains continue to be the main threat from Hurricane Ernesto. The Hurricane Center is forecasting 4 to 8 inches of rain to fall, increasing in the higher elevation of Belize. After landfall, the storm will take about a day to cross the Yucatan, and the terrain will diminish its winds. Once Ernesto re-emerges over water into the Bay of Campeche in the southern Gulf of Mexico, wind shear will be light and ocean waters warm with high heat content. Ernesto is then expected to redevelop some strength and potentially regain hurricane status while over water, which a few of the models are suggesting. Second landfall will probably occur Friday morning around Veracruz, Mexico, but could reach land anywhere from Tuxapan to Coatzacoalcos.

Ernesto closing in on the Yucatan

Tropical Storm Ernesto is closing in on the coast of the Yucatan Peninsula as a strong tropical storm with 65 mph winds. Latest data from the Air Force hurricane hunter aircraft in the storm found the pressure had dropped to its lowest value yet--988 mb at 9:12 am EDT. Top surface winds as seen by their SFMR instrument were in the 60 - 65 mph range, and the plane found 72 mph winds at their flight level of 5,000 feet, on the northeast side of the eye. Ernesto does not have an eyewall, but the Hurricane Hunters noted an eyewall may be the process of forming, from the north to the south-southeast side of the center. Visible and infrared satellite loops show that Ernesto's heavy thunderstorms have expanded in areal extent and intensity to form a Central Dense Overcast (CDO), a feature of intensifying tropical storms. Ernesto is encountering light wind shear of 5 - 10 knots. The dry air that mixed into Ernesto's core and disrupted it on Monday is no longer apparent on water vapor satellite loops.

Figure 1. True-color MODIS image from NASA's Terra satellite of Tropical Sotrm Ernesto, taken at 11:35 am EDT August 6, 2012. At the time, Ernesto had top winds of 65 mph. Image credit: NASA.

Forecast for ErnestoErnesto does not have an eyewall, and this will severely limit the storm's chances of rapid intensification until the storm can build one. However, Ernesto is under light wind shear and over warm ocean waters of 29°C with very high heat content, so some modest intensification to a Category 1 hurricane is possible before landfall occurs near midnight tonight near the Belize/Mexico border. Heavy rains will be the main threat from Ernesto. The storm will take about a day to cross the Yucatan, and its winds will probably diminish by 15 - 25 mph. Once Ernesto re-emerges over water into the Bay of Campeche in the southern Gulf of Mexico, wind shear will be light and ocean waters warm with high heat content. I expect Ernesto will increase its winds by 15 - 25 mph while over the Bay of Campeche, and the storm could be near Category 1 hurricane strength when it makes a second landfall near Veracruz, Mexico.

Crossing the Yucatan: a historyHurricanes and tropical storms regularly cross Mexico's Yucatan Peninsula and enter the Bay of Campeche, the Gulf of Mexico's southernmost region. Since the crossing usually takes less than a day and the peninsula is surrounded by warm ocean water that can help feed the storm during the crossing, the great majority of storms survive the trek. Once in the Bay of Campeche, most storms regenerate, even though there is not much room for the storm to go before a second landfall in Mexico occurs. This is because the curved shape of the mountain-lined coast helps boost counter-clockwise spin of the air, and the waters in the bay are among the warmest in the North Atlantic. Typically, a storm that crosses the Yucatan with a mostly westward track and enters the Bay of Campeche will intensify by 15 - 25 mph before making a second landfall in Mexico. Let's consider two historical analogue case for what might happen to Ernesto.

Figure 3. Tracks of all major hurricanes since 1851 near Mexico's Bay of Campeche. Karl was the most southerly major hurricane on record in the Gulf of Mexico. Image credit: NOAA Coastal Services Center.

Hurricane Karl of 2010Hurricane Karl of 2010 hit the Yucatan near the Belize/Mexican border as a tropical storm with 65 mph winds. Karl took 18 hours to cross the Yucatan, and weakened to a 45 mph tropical storm during the crossing. Remarkably, though, conventional and microwave satellite imagery indicated that the storm’s organization and vertical structure improved during the crossing, with the appearance of an eye-like feature and an increase in low-level spiral bands. This probably occurred as a result of frictional convergence--when air flowing over the smooth ocean surface moves over land, the increased friction causes the air to slow down and flow at a sharper angle towards a center of low pressure. Once the storm reached the Bay of Campeche, Karl took advantage of low wind shear, ocean temperatures of 29 - 30°C, and a moist atmosphere, to put on an impressive show of rapid intensification. Karl took only 12 hours to regain its strength, and within 36 hours of exiting the Yucatan, had intensified to a Category 3 hurricane with 125 mph winds. This sort of intensification so far south in the Bay of Campeche was unprecedented, and Karl was the strongest major hurricane ever observed so far south in the Gulf of Mexico. Karl dumped 10 -15 inches of rain over most of the northwestern half of the state of Veracruz, triggering floods that killed 22 and did $400 million in damage.

Figure 4. Track of Tropical Storm Hermine of 1980.

Tropical Storm Hermine of 1980A more likely historical analogue storm for Ernesto may be Tropical Storm Hermine of 1980. Hermine hit the Yucatan near the border between Belize and Mexico with 70 mph winds. The 24-hour crossing of the Yucatan weakened Hermine's winds to 50 mph. After emerging into the Bay of Campeche, Hermine turned to the west-southwest and made landfall southeast of Veracruz 30 hours later, with top winds of 70 mph.

Other storms of the past 30 years with a similar landfall location to Ernesto'sHurricane Dean of 2007 hit the Yucatan near the Belize/Mexican border as a large Category 5 hurricane with 175 mph winds. Dean took 10 hours to cross the Yucatan, and weakened to a 75 mph Category 1 hurricane during the crossing. Upon reaching the Bay of Campeche, Dean strengthened by 25 mph to a Category 2 hurricane with 100 mph winds, before its second landfall occurred.

92LA tropical wave in the far Eastern Atlantic (Invest 92L) is disorganized, with limited heavy thunderstorm activity and a modest amount of spin at mid-levels of the atmosphere. Of the six main models used operationally by NHC, only one--the HWRF--develops 92L. The storm is at least 6 days from the Lesser Antilles Islands, if it maintains a westward motion.

Figure 5. NOAA-19 AVHRR image of the big low pressure system in the Arctic, taken at 9 am EDT August 7, 2012. At the time, the GFS analysis gave a central pressure of 970 mb for the low. Image credit: NOAA and Environment Canada. Thanks go to wunderblogger Grothar for pointing out this image to me.

Big storm in the ArcticA remarkably intense low pressure system formed in the Arctic north of Alaska Monday, bottoming out with a central pressure of 963 mb at 2 pm EDT. A pressure this low is rare any time of the year in the Arctic, and is exceptionally so in summer. The storm is stacked vertically with the upper-level low, and will spin in place and slowly weaken over the next few days, but remain unusually strong. Strong winds behind the low's cold front caused a 1.3' storm surge Monday in Prudhoe Bay, on Alaska's north shore. As noted in Neven Acropolis' sea ice blog, the strong winds around this low have the potential to cause a large loss of Arctic sea ice, due to churning, increased wave action, pushing of ice into warmer waters, and the mixing up of warmer waters from beneath the ice. According to the latest analysis from the National Snow and Ice Data Center, Arctic sea ice extent was at a record low extent as of August 1. This week's big storm will likely keep Arctic sea ice at record low levels for the next week or two.

Angela Fritz will have an update on Ernesto late this afternoon or early this evening.

Ernesto nearing hurricane strength

Tropical Storm Ernesto is undergoing significant strengthening, and is not far from hurricane strength, according to data from this morning's Hurricane Hunter mission. The Air Force hurricane hunter aircraft in the storm found the pressure had dropped to 997 mb at 8:13 am EDT, and had dropped another 3 mb to 994 mb at 9:17 am. Surface winds as seen by their SFMR instrument had increased to 68 mph, and the plane found 89 mph winds at their flight level of 5,000 feet, on the northwest side of the eye. A full eyewall surrounding a small 9-mile diameter eye had formed. Ernesto's forward speed has slowed down to 12 mph--just half of what it was 24 hours ago, and this has allowed the surface center to align itself with the circulation at middle levels of the atmosphere. Visible satellite loops show that Ernesto's heaviest thunderstorms are now located near the center of the storm, and these thunderstorms have expanded in areal extent and intensity to form a Central Dense Overcast (CDO), a feature of intensifying tropical storms. Ernesto is still battling moderate wind shear of 10 - 15 knots, due to strong upper-level winds out of the west. Water vapor satellite loops show dry air to the west, but the environment around Ernesto is the moistest we've seen since it entered the Caribbean. On Sunday, Ernesto brought 1.73" of rain to Kingston, Jamaica, and top sustained winds of 37 mph.

Figure 1. Morning satellite image of Ernesto.

Forecast for ErnestoNow that Ernesto has slowed down in response to a trough of low pressure passing to the north, continued steady intensification appears likely. While wind shear is expected to remain in the moderate range the next two days, Ernesto is over warm ocean waters of 28°C with very high heat content, and rapid intensification to a Category 2 hurricane is possible today. The 8 am EDT SHIPS model forecast predicts a 26% chance of rapid intensification--a 30 mph increase in winds over a 24-hour period. The main obstacle to intensification will probably be proximity to land, as the center of Ernesto is likely to pass very close to the coast of Honduras late Monday night. This will put a portion of the storm's circulation over land, limiting intensification potential.

The winds this Monday morning at Puerto Lempira on the northeast coast of Honduras have not yet begun to increase, but will begin to rise this afternoon and peak near midnight tonight. High winds and heavy rains will spread westwards along the coast of Honduras early Tuesday morning, and reach coastal Belize near 2 pm Tuesday. If Ernesto survives its crossing of the Yucatan Peninsula, the potential exists for it to re-strengthen over the Bay of Campeche, and make a second landfall on Mexico's coast Thursday night south of Veracruz. However, most of the computer models predict that Ernesto will pop out so far south in the Bay of Campeche that the storm will have less than 24 hours over water. This makes significant re-intensification unlikely. I don't expect rain from Ernesto will get as far north as Brownsville, Texas.

Historic heat wave ends in OklahomaThe high temperature in Oklahoma City on Sunday reached 99°, snapping a string of 18 straight days the temperature had reached 100° or greater. The latest forecast calls for a few more days of temperatures in excess of 100° this week, but nothing like the heat wave last week that brought an unprecedented three straight days of 112° heat. The winds today in Oklahoma will be considerably lower than we saw over the weekend, which will aid firefighting efforts.

Ernesto disorganized; more fires, extreme heat for Oklahoma

A disorganized Tropical Storm Ernesto continues to speed westward at 23 mph across the Caribbean. Ernesto has brought sporadic heavy rains to Jamaica today, and Kingston has picked up 0.51" of rain as of noon, and recorded top sustained winds of 22 mph. Ernesto looks very unhealthy on visible satellite loops, with its low-level circulation center a naked swirl exposed to view with almost no heavy thunderstorm activity near the center. However, the storm does have some rather far-flung spiral bands, and these bands are bringing occasional heavy downpours to Haiti, western Cuba, Jamaica, and the southwest Dominican Republic. This morning's flight by the Hurricane Hunters found that Ernesto had a very high central pressure of 1006 mb and top winds near 50 mph. The latest wind shear analysis from the SHIPS model shows moderate shear of 10 - 15 knots affecting the storm, but there must be some wind shear the satellites are not able to detect affecting Ernesto, given its disorganized appearance. Water vapor satellite loops show a large area of dry air to the west, and this dry air is also interfering with Ernesto's organization.

Figure 1. Visible satellite image with 375 meter resolution taken of Ernesto by the new Suomi NPP satellite at 1:39 pm EDT August 4, 2012. At the time, Ernesto had a flare-up of intense thunderstorms, and had top winds of 60 mph. Image credit: University of Wisconsin Madison CIMSS.

Forecast for ErnestoErnesto's rapid forward speed of 23 mph has been part of the reason for its lack of intensification, but the storm is expected to slow down Monday and Tuesday in response to a trough of low pressure passing to the north. This slowing, in combination with low wind shear, a moister environment, and increasing heat energy in the ocean, may allow Ernesto to strengthen some before making landfall in Belize or Mexico's Yucatan Peninsula on Tuesday night. However, the storm will be passing very close to the north coast of Honduras, putting a portion of its circulation over land and limiting intensification potential. It is unlikely Ernesto will become a hurricane in the Caribbean; NHC is giving just a 19% chance that this will occur. The main threat from Ernesto will be heavy rains over Honduras, Belize, Mexico, and Jamaica. The track forecast for Ernesto has become a bit easier, since the storm's current disorganization and more southerly path make will make it more difficult for the storm to make a northwesterly turn into the Gulf of Mexico like the UKMET and GFDL models are predicting. A stronger Ernesto would have been more likely to turn northwest under the influence of a trough of low pressure passing to the north. If Ernesto survives its crossing of the Yucatan Peninsula, the potential exists for it to re-strengthen over the Bay of Campeche, and make a second landfall on Mexico's coast on Friday, between Tampico and Veracruz. It's pretty unlikely that Ernesto will hit the U.S.-- though Brownsville, Texas could see some rain from Ernesto's outer spiral bands on Friday, if the storm survives that long.

Figure 2. Morning satellite image of Ernesto, showing the exposed low-level center--a swirl due south of Jamaica--and only very limited heavy thunderstorm activity surrounding the center.

Tropical Storm Florence Tropical Storm Florence continues to plow westward at 14 mph over the Eastern Atlantic, and is not a threat to any land areas for next five days. The SHIPS model is predicting a moderate 5 - 15 knots of wind shear for Florence Sunday and Monday, but the shear will increase to the high range as Florence encounters an upper-level trough of low pressure on Tuesday. The latest Saharan Air Layer Analysis shows that a large area of dry air lies to the north and west of Florence, and this dry air will likely cause problems for the storm. Ocean temperature are near 26 - 26.5°C, which is right at the threshold for where a tropical storm can typically exist. It is possible that Florence could pose a threat to Bermuda next weekend, if the storm survives that long. Both the GFS and ECMWF models dissipate Florence before then.

Historic heat wave in OklahomaA second day of destructive fires affected Oklahoma on Saturday, thanks to extreme heat and drought, low humidities, and strong winds in advance of an approaching cold front. At 3 pm CDT Saturday, Oklahoma City had a temperature of 107°, a humidity of 19%, and winds of 16 mph gusting to 22 mph. The Oklahoma fires have destroyed at least 125 homes. The high temperature in Oklahoma City on Saturday reached 109°, the 12th warmest temperature recorded in the city since records began in 1891. Friday's high of 113° tied for the warmest temperature in city history.

The only comparable Oklahoma heat wave: August 1936The only heat wave in Oklahoma history that compares to the August 2012 heat wave occurred during the great Dust Bowl summer of 1936, the hottest summer in U.S. history. Oklahoma City experienced three days at 110° that summer, and a record streak of 22 straight days with a temperature of 100° or hotter. Those numbers are comparable to 2012's: three days at 110° or hotter, and a string of 18 consecutive days (so far) with temperatures of 100° or hotter. The weak cold front that passed though Oklahoma Saturday will bring temperatures about 10° cooler over the next few days, but high temperatures are still expected to approach 100° in Oklahoma City Sunday through Tuesday. It's worth noting that Oklahoma City has experienced only 11 days since 1890 with a high of 110° or greater. Three of those days were in 2011, three in 2012, and three in the great Dust Bowl summer of 1936.

Figure 4. Most of Oklahoma has experienced nine consecutive days with highs of 100° or more, and many regions, including Oklahoma City, have had a streak of eighteen such days. Image credit: Oklahoma Mesonet.

Severe thunderstorm complex forces evacuation of Lollapalooza A organized complex of severe thunderstorms developed over Eastern Iowa and Northern Illinois late Saturday afternoon, forming a dangerous bow echo that swept through Chicago, forcing the evacuation of the Lollapalooza music festival. NOAA's Storm Prediction Center (SPC) logged over 150 reports of wind damage from the storm, with five of the thunderstorms containing winds in excess of hurricane force (74 mph.) And just yesterday, my daughter was bemoaning her misfortune at not being able to get tickets to the show! The thunderstorm complex traveled about 300 miles from Eastern Iowa to Ohio, generating winds gust in excess of 58 mph along most of its path, meeting the definition of a derecho.

Figure 5. Radar image of the severe thunderstorm complex that spawned a dangerous bow echo over Chicago, which forced the evacuation of the Lollapalooza music festival.

Enigmatic Tropical Storm Ernesto continues westward across the Caribbean, but has weakened. Ernesto certainly looks impressive on visible satellite loops, with a symmetric shape, good spiral banding, and an upper-level outflow channel to the north and east. But this morning's flight by the Hurricane Hunters found that Ernesto had weakened, with top winds of just 50 mph, and a central pressure that had risen to 1008 mb. The storm is fighting low to moderate wind shear of 5 - 15 knots, and water vapor satellite loops show a large area of dry air to the west. Upper level winds from the west are driving this dry air into the west side of the storm. Ernesto's rains are staying just north of the ABC Islands, as seen on Aruba radar. The southern shore of the Dominican Republic is experiencing occasional heavy rains from Ernesto's spiral bands.

Figure 1. Morning visible satellite image of Ernesto taken at 8 am EDT, with echoes from a microwave satellite instrument in the 85 GHz band superimposed. Image credit: Navy Research Lab, Monterey.

Forecast for ErnestoErnesto continues to be a major challenge to forecast. Despite the seemingly favorable conditions for intensification expected today through Tuesday, with low wind shear, a moister environment, and increasing heat energy in the ocean, many of our top computer models refuse to predict intensification, and in fact, weaken the storm. Of the major dynamical models NHC uses operationally--the ECMWF, GFS, NOGAPS, UKMET, GFDL, and HWRF--only the NOGAPS and GFDL show Ernesto reaching hurricane strength in the Caribbean. The ECMWF dissipates the storm. However, some of the best statistical models, such as the LGEM and SHIPS, do show Ernesto becoming a Category 1 or 2 hurricane in the Caribbean. The official NHC intensity forecast of a Category 1 hurricane between Jamaica and Mexico's Yucatan Peninsula is a reasonable compromise, but the uncertainty in this is high. It would not be a surprise to see Ernesto mysteriously degrade, or undergo rapid intensification into a Category 2 hurricane off the coast of the Yucatan. Such is the state of modern hurricane intensity forecasting. Given that we don't have a very good idea of how strong Ernesto will become, making an accurate track forecast is hard. A stronger Ernesto will be more likely to feel the influence of a trough of low pressure moving to the north of the storm on Tuesday, which would pull the storm to the northwest into the Gulf of Mexico. This would likely result in a landfall in the U.S. A weaker Ernesto is more likely to head almost due west, resulting in a landfall Wednesday in Belize or Mexico's Yucatan Peninsula. This is the more likely solution, given the recent behavior of the storm.

Tropical Storm Florence formsTropical Storm Florence has arrived in the far Eastern Atlantic, marking the 3rd earliest date for formation of the Atlantic's sixth named storm. Only 2005 and 1936 had earlier arrivals of the season's sixth storm. The new tropical storm developed unusually quickly from a tropical wave that moved off the coast of Africa just two days ago, and the storm's formation was aided by a pulse warm ocean water and associated low pressure called a Convectively Coupled Kelvin Wave (CCKW.) The SHIPS model is diagnosing a moderate 10 - 15 knots of wind shear over Florence, and predicts that the shear will stay in the moderate range over weekend, then increase to the high range as Florence encounters an upper-level trough of low pressure. The latest Saharan Air Layer Analysis shows that a large area of dry air lies to the north and west of Florence, and this dry air will likely cause problems for the storm. Ocean temperature are near 26 - 26.5°C, which is right at the threshold for where a tropical storm can typically exist. The predicted steering current flow for Florence does not favor a long-range threat to any land areas.

Historic heat wave in OklahomaA historic heat wave and drought fueled raging fires on Friday in Oklahoma. The fires destroyed at least 65 homes, forced multiple evacuations, and closed major roads. Oklahoma City had its hottest day in history, hitting 113°, tying the city's all-time heat record set on August 11, 1936. The low bottomed out at 84°, the warmest low temperature ever recorded in the city (previous record: a low of 83° on August 13, 1936.) Oklahoma City has now had three consecutive days with a high of 112° or higher, which has never occurred since record keeping began in 1891. With today's high expected to reach 113° again, the streak may extend to four straight days. Yesterday was the third consecutive day with more than a third of Oklahoma experiencing temperatures of 110° or higher, according to readings from the Oklahoma Mesonet. NOAA's Storm Prediction Center (SPC) declared a "Critical" fire weather day over most of Oklahoma yesterday, due to extreme heat and drought, low humidities, and strong winds. Between 4 - 5 pm CDT Friday, Oklahoma City had a temperature of 113°, a humidity of 12%, and winds of 14 mph gusting to 25 mph. Another "Critical" fire weather day has been declared for Saturday. A cold front approaching from the northwest will bring winds even stronger than Friday's winds, and Oklahoma will likely endure another hellish day of extreme heat, dryness, and fires.

Only comparable heat wave: August 1936The only heat wave in Oklahoma history that compares to this week's occurred in the great Dust Bowl summer of 1936, the hottest summer in U.S. history. Oklahoma City experienced three days at 110° that summer, and a record streak of 22 straight days with a temperature of 100° or hotter. Those numbers are comparable to 2012's: three days at 110° or hotter, and a string of 17 consecutive days with temperatures of 100° or hotter. It's worth noting that Oklahoma City has experienced only 11 days since 1890 with a high of 110° or greater. Three of those days were in 2011, three were in 2012, and three were in the great Dust Bowl summer of 1936. Clouds moved in over Tulsa, Oklahoma yesterday, holding down the high temperature to just 107°, ending that city's 3-day streak of 110°+ days. The only longer streak was 5 consecutive days on August 9 - 13, 1936.

Figure 3. Most of Oklahoma has experienced eight consecutive days with highs of 100° or more, and many regions, including Oklahoma City, have had a streak of 17 such days. Image credit: Oklahoma Mesonet.

Ernesto hit the Windward Islands; 128.5°F in Kuwait

Tropical Storm Ernesto lashed the Windward Islands with strong winds and heavy rain early this morning, as it passed over St. Lucia near 7 am AST. Ernesto brought sustained winds of 43 mph to Barbados at 7 am AST, and sustained winds of 41 mph, gusting to 63 mph to St. Lucia at 6:15 am AST. Ernesto looks moderately well-organized on Martinique radar, with spiral bands to the north and south feeding into an echo-free center. Ernesto is beginning to show more organization on visible satellite loops, with the very limited heavy thunderstorm activity near its center now expanding, spiral banding increasing, and an upper-level outflow channel developing to the north. Ernesto is fighting moderate wind shear of 10 - 20 knots, and water vapor satellite loops show a large area of dry air to the west. Strong upper level winds from the west are driving this dry air into the core of the storm, disrupting it. It appears, though, that Ernesto has fended off the most serious challenges to its survival, as the pressure has fallen to 1002 mb, and the storm's appearance on radar and satellite is gradually improving. The latest 7:30 am center report from the Hurricane Hunters also indicated that Ernesto was beginning to build an eyewall.

Forecast for ErnestoErnesto's survival into today means that the storm now potentially poses a formidable threat to the Western Caribbean. Wind shear is expected to drop to the low range, 5 - 10 knots, later today, and remain low for the next five days, according to the 8 am EDT run of the SHIPS model. With the storm entering a moister environment with increasing heat energy in the ocean, the official NHC forecast of Ernesto reaching hurricane strength by early Monday morning near Jamaica is a reasonable one. The reliable computer models predict a west to west-northwest motion through the Caribbean, with the storm's heavy rains staying south of Puerto Rico and the Dominican Republic. The more southerly path predicted by the usually reliable ECMWF model, which brings Ernesto to a landfall in Belize on Wednesday, is being weighted less heavily by NHC, since they are assuming Ernesto will stay stronger than the ECMWF model is forecasting. Once Ernesto enters the Central Caribbean on Sunday, the storm's outer spiral bands will likely cause flooding problems in Southwest Haiti, Jamaica, and the Cayman Islands. Of the major dynamical models NHC uses operationally--the ECMWF, GFS, NOGAPS, UKMET, GFDL, and HWRF--none clearly show Ernesto reaching hurricane strength in the Caribbean. However, some of the best statistical models, such as the LGEM and SHIPS, do show Ernesto becoming a hurricane in the Caribbean. By Monday, a trough of low pressure passing to the storm's north may be capable of turning Ernesto more to the northwest, resulting in the storm entering the Gulf of Mexico by the middle of next week.

New African tropical disturbance 90LA strong tropical wave is located just off the coast of Africa, about 175 miles south of the Cape Verde Islands. This disturbance, designated Invest 90L by NHC Friday morning, is headed west-northwest at 10 - 15 mph. Wind shear is a high 20 - 30 knots over 90L, but is expected to drop, and water temperatures are warm enough to support development. NHC gave 90L a 30% chance of developing into a tropical depression by Sunday morning in their 8 am Friday Tropical Weather Outlook.

Kuwait hits 53.6°C (128.5°F): 2nd hottest temperature in Asian historyAn extraordinary high temperature of 53.6°C (128.5°F) was recorded in Sulaibya, Kuwait on July 31, the hottest temperature in Kuwait's history, and the 2nd hottest temperature ever measured in Asia. According to weather records researcher Maximiliano Herrera, Sulaibya is in a location well-suited for recording extreme high temperatures, since high sand dunes surround the site, keeping the wind low and hampering sea breezes from cooling the city. Most record books list the 54°C (129.2°F) recorded on 21 June 1942 in Tirat Zvi Israel as the site of Asia's all-time maximum temperature, but this record is disputed. The previous second warmest temperature in Asian history was set 53.5°C (128.3°F) at MohenjuDaro, Pakistan on May 26, 2010.

Extreme heat in OklahomaThe most intense and widespread heat wave in Oklahoma since August, 1936 brought more than half of the state temperatures of 110° or higher for the second consecutive day on Thursday. The temperature at the Oklahoma City airport hit 112°, for the 2nd day in a row. These are the city's 2nd highest temperatures since record keeping began in 1890. The only hotter day was August 11, 1936, when the temperature hit 113°. Thursday's temperatures in Oklahoma were generally a degree or two cooler than Wednesday's, with the hottest temperature reported a 116° reading from a location just south of Tulsa International Airport. The highest reading Thursday at any major airport was a 114° temperature at Tulsa Jones Airport. Oklahoma's all-time state record is 120°, set in Tipton on June 27, 1994, and at three locations in 1936. Freedom, in the northwest part of the state, hit 121° on Wednesday, but this reading will need to be reviewed to see if the sensor was properly sited.

Tropical Storm Ernesto forms

Observations from an Air Force hurricane hunter aircraft indicate that Tropical Storm Ernesto has arrived. The season's fifth tropical storm has surface winds of 50 mph on the north side near 14°N, but has an ill-defined surface circulation with almost no winds out of the west. Ernesto is not a pretty sight on visible satellite loops, with patchy heavy thunderstorm activity and little spiral banding. Arc-shaped low clouds can be seen racing away from Ernesto on its north side, an indication that the storm is ingesting dry air that is causing strong thunderstorm downdrafts. This sort of phenomena is seen in storms that are struggling to hold together in the face of wind shear and dry air. Wind shear over Ernesto is at the moderate level, 10 - 20 knots. Water vapor satellite loops show that Ernesto is at the southern edge of a large area of dry air. Wind shear due to strong upper level winds from the west is driving this dry air into the core of the storm, disrupting it. A series of Air Force hurricane hunter aircraft are scheduled to visit Ernesto every six hours to keep tabs on it.

Figure 1. Afternoon satellite image of Ernesto. Note the arc-shaped low clouds on the north side of Ernesto, which mark the boundaries of where thunderstorm outflow due to ingestion of dry air is occurring.

Forecast for ErnestoSince the Hurricane Hunters found Ernesto's strongest winds to be near 14°N, I expect that Martinique, Dominica, St. Lucia, and Barbados will see the strongest winds on Friday during Ernesto's passage through the islands. Wind shear is expected to remain moderate, 10 - 20 knots, through Friday, then fall to the low range through Tuesday, according to the 2 pm EDT run of the SHIPS model. Given Ernesto's poor organization, I give a 20% chance that the storm will degenerate into a tropical wave on Friday. If this happens, the storm will still be capable of bringing winds of 50 mph to the Windward Islands, though--and will still be capable of regenerating into a tropical storm in the Western Caribbean. Once Ernesto clears the Lesser Antilles, the reliable computer models predict a west to west-northwest motion through the Caribbean, with the storm's heavy rains staying south of Puerto Rico and the Dominican Republic. The more westerly path predicted by the usually reliable ECMWF model, which brings Ernesto to a landfall in Honduras on Monday night, is being discounted by NHC, since they are assuming Ernesto will stay stronger than the ECMWF model is forecasting. Once Ernesto enters the Central Caribbean on Sunday, it is possible that the storm's outer spiral bands will cause flooding problems in Southwest Haiti, Jamaica, and the Cayman Islands. Of the major models NHC uses operationally--the ECMWF, GFS, NOGAPS, UKMET, GFDL, and HWRF--only the HWRF clearly shows Ernesto reaching hurricane strength over the next five days. So, the official NHC forecast of a hurricane near Jamaica on Monday is an aggressive one. By Monday, a trough of low pressure passing to the storm's north may be capable of turning Ernesto more to the northwest, resulting in the storm entering the Gulf of Mexico by the middle of next week. Stay tuned.

Figure 2. Radar image taken just before landfall as Typhoon Damrey approached the coast of China. Image credit: weather.com.cn.

Typhoon Damrey hits ChinaTyphoon Damrey hit China near the Chenjiagang Township of Xiangshui County around 9:30 p.m. local time Thursday as a strong Category 1 storm with winds near 85 mph. It is too early to know what sort of damage the storm may have caused. The Western Pacific is entering a very active period, and is likely to get at least two more new named storms in the next week.

TD 5 strugging; 121° in Oklahoma; little change to U.S. drought

A disheveled Tropical Depression Five is clinging to life in the face of stiff wind shear and dry air, as the storm heads towards the Lesser Antilles Islands with a forward speed of 20 mph. The depression is very ragged looking on visible satellite loops. The depression has only a small area of heavy thunderstorms, which are on the south side of the center due to high wind shear and dry air on the northern side of the storm. Wind shear over TD 5 is at the moderate level, 15 - 20 knots. Water vapor satellite loops show that TD 5 is at the southern edge of a large area of dry air. Wind shear due to strong upper level winds from the west is driving this dry air into the core of the storm, disrupting it. The first Air Force hurricane hunter aircraft will investigate TD 5 Thursday afternoon to give us a better idea of its strength.

Figure 1. Morning satellite image of TD 5.

Forecast for TD 5TD 5's west to west-northwest motion should bring its outer rain bands to Barbados early Friday morning, and high winds and heavy rain will spread over the rest of the Windward Islands by Friday afternoon. Since wind shear will be higher on the storm's north side, I expect the heaviest weather will be on the south side of TD 5, over the Windward Islands. Wind shear is expected to remain moderate, 10 - 20 knots, through Friday, according to the 8 am EDT run of the SHIPS model. However, the reliable ECMWF model predicts higher shear on Friday and Saturday, which could tear TD 5 apart. A number of our reliable computer models predict TD 5 will not survive its passage through the islands this weekend. However, most of the models agree that once TD 5 (or its remnants) pass enter the Central Caribbean on Monday, wind shear will fall to the low range, and strengthening (or regeneration) can occur. Once TD 5 enters the Western Caribbean between Jamaica and Mexico's Yucatan Peninsula, a trough of low pressure passing to the storm's north may be capable of turning TD 5 more to the northwest, resulting in the storm entering the Gulf of Mexico. An equally likely scenario at this point is that TD 5 will not turn, and instead will take a more westerly path over the Yucatan Peninsula. The long-term forecast for what the storm will do once it gets to the Western Caribbean depends upon whether or not TD 5 survives during the next two days.

Extreme heat in OklahomaWednesday was the hottest day in Oklahoma since August 1936, said wunderground's weather historian, Christopher C. Burt, with more than half of the state recording temperatures of 110° or higher. The hot spot Wednesday: 121° in Freedom, in the northwest part of the state. If verified, this temperature would beat Oklahoma's all-time state temperature record of 120°, set in Tipton on June 27, 1994, and at three locations in 1936. The official temperature at the Oklahoma City airport hit 112° Wednesday, tied for the city's 2nd highest temperature since record keeping began in 1890. The only hotter day was August 11, 1936, when the temperature hit 113°. Other hot spots in Oklahoma yesterday:

118° at station W1DY (just north of Oklahoma City's Wiley Post Airport)116° at Claremore 115° at Chandler, Enid, Guthrie, Okmulgee, Omega, and Kingfisher

It was also brutally hot in northern Texas, with the hottest temperature a 118° reading at station DW3597 in Wichita Falls, Texas. This is just shy of the state record of 120° set on August 12, 1936, in Seymour. The Wichita Falls airport hit 112°, which was just 1° below the all-time hottest August temperature ever measured in the city (113° on August 8, 1964.)

Little change to the U.S. drought during the past weekThe great U.S. drought of 2012 remained about the same size and intensity over the past week, said NOAA in their weekly U.S. Drought Monitor report issued Thursday, August 2. The area of the contiguous U.S. covered by drought dropped slightly, from 64% to 63% over the past week, and the area covered by severe or greater drought stayed constant at 46%. If we average the past five drought monitor reports to come up with unofficial July drought numbers for the contiguous U.S., the 2012 drought is second only to the great Dust Bowl drought of July 1934 in terms of the area of the contiguous U.S. covered by moderate or greater drought:

The U.S. Department of Agriculture has now declared more than half of all counties in the U.S. (50.3%) natural disaster areas due to the drought, adding 218 counties in 12 states to its disaster list on Wednesday. In all, nearly 1,600 counties in 32 states have been declared disaster areas.

Figure 3. July 31, 2012 drought conditions showed historic levels of drought across the U.S., with 64% of the contiguous U.S. experiencing moderate or greater drought, and 46% of the county experiencing severe or greater drought. Image credit: U.S. Drought Monitor.

Figure 4. The twice-monthly U.S. Drought Outlook, updated on Thursday, August 2, predicts that drought will continue through October over most of the U.S., Some expansion to the north and south is expected, but also some improvement over the Eastern U.S. and Southwest U.S. Image credit: NOAA Climate Prediction Center.

Drought predicted to extend into OctoberIn their twice-monthly drought outlook, released on Thursday August 2, NOAA's Climate Prediction Center had this to say about the Midwest drought: Unfortunately, the self-perpetuation of regional drought conditions, with very dry soils and very limited evapotranspiration, tends to inhibit widespread development of or weaken existing thunderstorm complexes. It would require a dramatic shift in the weather pattern to provide significant relief to this drought, and most tools and models do not forecast this. Unfortunately, all indicators (short and medium-term, August, and August-October) favor above normal temperatures. With much of the Plains already in drought and getting worse, above normal temperatures expected into the fall, and a dry short-term and 30-day forecast, the drought should persist, with some possible development in the northern Plains.

One bright spot: drought conditions are expected to improve over the Southwest U.S. over the next few weeks, as the annual summer monsoon peaks and brings heavy rains. The Southeast U.S. has seen some improvement over the past week. The potential for a landfalling tropical storm to bring drought-busting rains during the August - September - October peak of hurricane season led NOAA to predict possible improvement in drought conditions over the Southeast U.S.

Drought already creating global ripplesThe U.S. is the world's largest exporter of corn and wheat, and 3rd largest exporter of soybeans. According to the Christian Science Monitor, food price increases due to the U.S. drought is already causing unrest in other parts of the world: "Take Indonesia, where soybeans are used to make tofu, the staple protein for the country's poor. There, soybean prices have risen 33 percent in the past month, and are already causing tensions. On July 26, there were clashes in Jakarta and other major cities in markets as a coalition of tofu producers sought to enforce a national production strike protesting against a 5 percent soybean import duty."

Tropical Storm Watches are flying for much of the Lesser Antilles, as the islands await the arrival of Tropical Depression Five, which formed at 5 pm EDT today. The new depression is still fairly ragged looking, as seen on visible satellite loops. Heavy thunderstorm activity is only on the south side of the center, due to higher wind shear on the northern side of the storm. Wind shear over TD 5 is at the moderate level, 10 - 15 knots. TD 5 has one respectable low-level spiral band on its south side, but additional spiral bands are beginning to appear, and the areal coverage of the storm's heavy thunderstorms has increased markedly in the late afternoon hours. Water vapor satellite loops show that TD 5 has a reasonably moist environment. Ocean temperatures are 28°C, (82°F) which is about 0.5°C above average for this time of year. The first Air Force hurricane hunter aircraft will investigate TD 5 Thursday afternoon to give us a better idea of its strength.

Figure 1. Afternoon satellite image of TD 5.

Forecast for TD 5TD 5's west-northwest motion should bring its outer rain bands to Barbados early Friday morning, and high winds and heavy rain will spread over the rest of the Windward Islands by Friday afternoon. Since wind shear will be higher on the storm's north side, I expect the heaviest weather will be on the south side of TD 5, over the Windward Islands. Wind shear is expected to remain moderate, 10 - 15 knots, through Friday, ocean temperatures will remain near 28°C, and mid-level moisture will be a moderate 60 - 70%, according to the 2 pm EDT run of the SHIPS model. This should allow TD to become Tropical Storm Ernesto by Thursday. NHC is giving a 27% chance that TD 5 will become a hurricane by Friday afternoon, when it will be passing through the islands. The reliable computer models are not in good agreement on the future intensity of TD 5. A band of high wind shear of 20 - 40 knots associated with an upper level low lies to the north of TD 5, and our most reliable model, the ECMWF, predicts that this shear will extend down into the islands on Friday and Saturday, and tear TD 5 apart. However, the almost equally reliable GFS model predicts that this band of wind shear will remain north of TD 5, and the storm will have clear sailing for the next five days, with only moderate shear of 10 - 15 knots affecting it. At this point, we'll have to wait and see how future model runs handle the shear forecast.

Typhoon Saola hits TaiwanIn the Western Pacific, typhoon season is in full swing with two typhoons expected to make landfall in China on Thursday. Typhoon Saola hit northern Taiwan as a Category 2 storm with 105 mph winds near 3 am local time on August 2, 2012. Saola is predicted to hit mainland China 300 miles south of Shanghai on Thursday as a Category 1 typhoon. Typhoon Damrey, a Category 1 storm located just south of Korea, is expected to hit China about 150 miles north of Shanghai on Thursday as a Tropical Storm.

Extreme heat in OklahomaThe withering heat in Oklahoma continued on Wednesday, with the prize for most ridiculous heat a 115° temperature recorded in Okmulgee, Oklahoma. This is not far from Oklahoma's state temperature record of 120°, set in in Tipton on June 27, 1994. Oklahoma City has hit 110° thus far this afternoon, which is the 3rd highest temperature measured in the city since record keeping began in 1890. The only hotter days were August 10 - 11, 1936, when the temperature hit 112° and 113°, respectively. Chandler, Enid, Guthrie, Norman, Chickasha, and Shawnee in Oklahoma have all hit 113° today.

Disturbance 99L more organized; record melting in Austrian Alps

A tropical wave (Invest 99L) near 11°N 47°W, about 1000 miles east of the Lesser Antilles Islands, is showing increasing organization, and has the potential to develop into a tropical depression today or tomorrow as it moves westward at 15 - 20 mph. Visible satellite loops show that the disturbance now has two respectable low-level spiral bands, one to the north and one to the south, and a moderate amount of heavy thunderstorms near the center. The thunderstorm activity has not changed much in intensity this morning. A well-defined surface circulation is not evident on satellite images, but last night's 8:30 pm EDT pass from the ASCAT satellite showed a broad, elongated center with light winds had formed. Water vapor satellite loops show that 99L has a reasonably moist environment, and the latest Saharan air layer analysis shows that the dry air from the Sahara is not present in large quantities over the central tropical Atlantic. WInd shear over the disturbance has increased some since Tuesday, and is now at the moderate level, 10 - 15 knots. Ocean temperatures are 28°C, (82°F) which is about 0.5°C above average for this time of year.

Figure 1. Morning satellite image of Invest 99L.

Figure 2. Vertical instability over the tropical Atlantic in 2012 (blue line) compared to average (black line.) The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere. Thunderstorms grow much more readily when vertical instability is high. Instability has been lower than average, due to an unusual amount of dry, sinking air in the atmosphere, reducing the potential for tropical storm formation. Image credit: NOAA/NESDIS/CIRA.

Forecast for 99LWind shear is expected to remain light to moderate, 5 - 15 knots, through Friday, ocean temperatures will remain near 28°C, and mid-level moisture will be a moderate 60 - 70%, according to the 8 am EDT run of the SHIPS model. The disturbance has gained a bit of latitude and is now at 11°N, which will help it leverage the Earth's spin more to acquire its own spin. These conditions are probably sufficient for 99L to become Tropical Depression Five, with Thursday being the most likely day for this to happen. However, the reliable computer models are not very eager to develop 99L, and none show it becoming a hurricane over the next five days. This is probably because the atmosphere over the tropical Atlantic is unusually stable for this time of year (Figure 2), with large-scale areas of dry, sinking air present. Climatologically, we see very few Cape Verdes-type hurricanes forming near the Lesser Antilles Islands this early in August, and I expect 99L will struggle at times over the next few days. This is particularly likely if 99L goes north of 13°N, where a band a high wind shear of 20 - 40 knots associated with the subtropical jet stream lies. At 8 am Wednesday, NHC gave 99L a 60% chance of developing into a tropical depression by Friday morning. Residents and visitors to the Lesser Antilles Islands should anticipate heavy rains and strong winds from 99L beginning to affect the islands as early as Friday morning. The long-range fate of 99L next week is uncertain, but a track west to west-northwest through the Caribbean is the most popular solution from the models.

Two typhoons headed towards ChinaIn the Western Pacific, typhoon season is in full swing with two typhoons headed towards China. The more dangerous of the two is Category 2 Typhoon Saola, which is predicted to skirt the northern coast of Taiwan and hit mainland China 300 miles south of Shanghai on Friday as a Category 3 typhoon. Typhoon Damrey, a Category 1 storm located just south of Japan, is expected to hit China about 150 miles north of Shanghai on Thursday at Category 1 strength.

Extreme heat in OklahomaThe withering heat in America's heartland continued on Tuesday, with the prize for most ridiculous heat a 113° temperature recorded in Chandler, Oklahoma. A close second: Tulsa hit 112°, just 3° below the city's all-time high of 115° set on August 10, 1936. The low temperature in Tulsa was 88° Tuesday morning, tying the record for warmest low temperature in city history set just the previous day. Six locations in Oklahoma hit 112° or hotter Tuesday, and the forecast calls for highs near 112° again today over portions of Oklahoma.

Extreme dryness in the Central U.S.A few final tallies for July precipitation are in, and several U.S. cities in the heart of the drought region set new records for driest July:

Record early snow melt in the Austrian AlpsOne of the longest meteorological data records at high altitude comes from Sonnblick, Austria, on a mountaintop in the Alps with an elevation of 3106 meters (10,200 feet.) The observatory typically sees maximum snow depths of 3 - 4 meters (10 - 13 feet) during winter. According to weather records researcher Maximiliano Herrera, the snow had never completely melted at Sonnblick until the summer of 1992. Complete snow melt did not happen again until August 12, 2003, and has happened an average of once every two years since then--but always in September. Yesterday, on July 31, the snow completely melted at Sonnblick, the earliest melting since record keeping began in 1886. It's been an exceptionally hot summer in Austria, which experienced its 6th warmest June since record keeping began in 1767. Sonnblick Observatory recorded its all-time warmest temperature of 15.3°C (60°F) on June 30. Vienna hit 37.7°C (100°F) that day--the hottest temperature ever measured in June in Austria. Note that the two mountains in the Alps with long climate records, Saentis in Switzerland and Zugspitze in Germany, beat their records for earliest melting last year in 2011 (Saentis beat the previous record of 2003, and Zugspitze tied the record set in 2003.)